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THE LIBRARY 

OF 

THE UNIVERSITY 
OF CALIFORNIA 



PRESENTED BY 

PROF. CHARLES A. KOFOID AND 
MRS. PRUDENCE W. KOFOID 




PART II. 

THE HISTORY OF THE MAMMALIAN 
EMBRYO. 



F.&B. 



INTRODUCTION. 



THE most important difference between the de- 
velopment of Mammalia and Aves depends upon the 
amount and distribution of the food-yolk in the ovum. 
In birds, as we have seen (Ch. i.), the ovum is large and 
the greater part of it so heavily charged with food-yolk 
that it is unable to segment. The segmentation is con- 
fined to one small portion, the germinal disc, the pro- 
toplasm of which is less burdened with food-yolk than 
that of the remainder of the ovum. Such partial seg- 
mentation is known as meroblastic. 

In Mammals, on the other hand, the ovum is small 1 , 
and contains but a slight amount of food-yolk ; the little 
there is being distributed uniformly throughout. In con- 
sequence of this the whole ovum is able to segment ; the 
segmentation therefore belongs to the holoblastic type. 
This fundamental difference in the constitution of the 
ovum of Birds and Mammals is accompanied not only by 
differences in the segmentation but also by impoifcant 
differences, as we shall see, in the stages of development 
which immediately follow segmentation. Finally, in 

1 The human ovarian ovum is T ^ T to 1 | ir of an inch in diameter. 

202 



308 INTRODUCTION. 

birds, as we have seen, the nutrition of the developing 
embryo is entirely effected at the expense of the food- 
yolk and albumen with which the ovum was charged 
in the ovary and oviduct respectively, and the eggs 
leave the parent very soon after the close of segmenta- 
tion. In the Mammalia the absence of sufficient food- 
yolk necessitates the existence of some other source of 
nutriment for the embryo, and that source is mainly the 
maternal blood. 

The development of Mammalia may be divided into 
two periods : 1. the development within the uterus ; 2. 
the development after birth. 

In all the higher Mammalia the second period is very 
unimportant, as compared with the first ; for the young 
are born in a condition closely resembling that of the 
adult of the species to which they belong. The de- 
velopment during the first period takes place in the 
uterus of the mother, and nutriment passes from the 
maternal blood to that of the embryo by means of a 
structure, to be described in detail hereafter, known as 
the placenta. This difference between the development 
of Birds and Mammals may be briefly expressed by saying 
that the former are oviparous, while the latter are vivi- 
parous. 

The source of nutriment during the second period 
is the Mammary glands. In certain of the lower Mam- 
malia (Marsupials) the young are born in a very im- 
mature condition, and become attached by their mouths 
to the nipples of these glands. They are carried 
about, usually in a special pouch (marsupium) by the 
mother, and undergo in this position the greater part of 
the remainder of their development. 



CHAPTER X. 

GENERAL DEVELOPMENT OF THE EMBRYO. 

THERE is a close agreement in the history of the 
development of the embryo of the various kinds of 
Mammals. We may therefore take one, the Rabbit, as 
a type. There are without doubt considerable varia- 
tions to be met with in the early development even of 
species nearly allied to the Rabbit, but at present the 
true value of these variations is not understood, and 
they need not concern us here. 

The ovarian ovum. Mammals possess two ovaries 
situated in the body cavity, one on either side of the 
vertebral column immediately posterior to the kidneys. 
They are somewhat flattened irregularly oval bodies, a 
portion of the surface being generally raised into pro- 
tuberances due to projecting follicles. 

In an early stage of development the follicle in the 
mammalian ovary is similar to that of the fowl, and is 
formed of flat cells derived from the germinal cells ad- 
joining the ovum. As development proceeds however 
it becomes remarkably modified. These flat cells sur- 
rounding the ovum become columnar and then one or 
two layers deep. Later they become thicker on one 
side of the ovum than on the other, and there appears 



310 THE MAMMALIAN EMBRYO. [CHAP. 

in the thickened mass a cavity which gradually becomes 
more and more distended and filled with an albuminous 
fluid. 

As the cavity enlarges, the OYum, around which are 
several layers of cells, forms a prominence projecting 
into it. The follicle cells are known as the membrana 
granulosa, and the projection in which the ovum lies as 
the discus or cumulus proligerus. The whole structure 
with its tunic is known as the Graafian follicle. 

If the ovary of a mature female during the breeding 
season be examined, certain of the protuberances on its 
surface maybe seen to be considerably larger than others; 
they are more transparent than their fellows and their 
outer covering appears more tense ; these are Graafian 
follicles containing nearly or quite ripe ova. Upon pierc- 
ing one of these follicles with a needle-point the ovum 
contained therein spirts forth together with a not incon- 
siderable amount of clear fluid. 

Egg Membranes. The ovum is surrounded by a 
radiately striated membrane, the zona radiata, internal 
to which in the nearly ripe egg a delicate membrane 
has been shown, by Ed. v. Beneden, to exist. The cells 
of the discus are supported upon an irregular granular 
membrane external to the zona radiata. This mem- 
brane is more or less distinctly separated from the zona, 
and the mode of its development renders it probable 
that it is the remnant of the first formed membrane 
in the young ovum and is therefore the vitelline mem- 
brane. 

Maturation and impregnation of the ovum. As 
the ovum placed in the Graafian follicle approaches 
maturity the germinal vesicle assumes an excentric 



X.] IMPKEGNATION. 311 

position and undergoes a series of changes which have 
not been fully worked out, but which probably are of 
the same nature as those which have been observed in 
other types (p. 17). The result of the changes is the 
formation of one or more polar bodies, and the nucleus 
of the mature ovum (female pronucleus). 

At certain periods one or more follicles containing a 
ripe ovum burst 1 , and their contents are received by 
the fimbriated extremity of the Fallopian tube which 
appears according to Hensen to clasp the ovary at the 
time. The follicle after the exit of the ovum becomes 
filled with blood and remains as a conspicuous object on 
the surface of the ovary for some days. It becomes 
eventually a corpus luteum. The ovum travels slowly 
down the Fallopian tube. It is still invested by the 
zona radiata, and in the rabbit an albuminous envelope 
is formed around it in its passage downwards. Im- 
pregnation takes place in the upper part of the Fallo- 
pian tube, and is shortly followed by the segmentation, 
which is remarkable amongst the Amniota for being 
complete 2 . 

The entrance of the spermatozoon into the ovum 
and its subsequent fate have not been observed. Van 
Beneden describes in the rabbit the formation of the 
first segmentation nucleus (i.e. the nucleus of the ovum 
after fertilization) from two nuclei, one peripheral and 
the other ventral, and deduces from his observations 

1 So far as is known there is no relation between the bursting of 
the follicle and the act of coition. 

2 It is stated by Bischoff that shortly after impregnation, and 
before the commencement of the segmentation, the ova of the rabbit 
and guinea-pig are covered with cilia and exhibit the phenomenon of 
rotation. This has not been noticed by other observers. 



312 



THE MAMMALIAN EMBRYO. 



[CHAP 



that the peripheral nucleus was derived from the sper- 
matic element. 

Segmentation. The process of segmentation oc- 
cupies in the rabbit about 72 hours; but the time of 
this and all other stages of development varies con- 
siderably in different animals. 

The details of segmentation in the rabbit are differ- 
ently described by various observers ; but at the close of 
segmentation the ovum appears undoubtedly to be 
composed of an outer layer of cubical hyaline cells, 
almost entirely surrounding an inner mass of highly 
granular rounded or polygonal cells. 

FIG. 95. 




OPTICAL SECTIONS OF A RABBIT'S OVUM AT TWO STAGES 
CLOSELY FOLLOWING UPON THE SEGMENTATION. 

(After E. van Beneden.) 

ep. outer layer ; %. inner mass ; bp. Van Beneden's blastopore. 
The shading of the outer and inner layers is diagrammatic. 

In a small circular area however the inner mass of 
cells remains exposed at the surface (Fig. 95, A). This 



X.] SEGMENTATION. 313 

exposed spot may for convenience be called with v. Bene- 
den the blastopore, though, as will be seen by the ac- 
count given of the subsequent development, it in no 
way corresponds with the blastopore of other vertebrate 
ova. 

In the following account of the segmentation of the rabbit's 
ovum, v. Beneden's description is followed as far as the details 
are concerned, his nomenclature is however not adhered to 1 . 

According to v. Beneden the ovum first divides into two 
nearly equal spheres, of which one is slightly larger and more 
transparent than the other. The larger sphere and its products 
will be spoken of as the outer spheres, and the smaller one 
and its products as the inner spheres, in accordance with their 
different destinations. 

Both the spheres are soon divided into two, and each of the 
four so formed into two again ; and thus a stage with eight 
spheres ensues. At the moment of their first separation these 
spheres are spherical, and arranged in two layers, one of them 
formed of the four outer, and the other of the four inner spheres. 
This position is not long retained, for one of the inner spheres 
passes to the centre ; and the whole ovum again takes a spherical 
form. 

In the next phase of segmentation each of the four outer 
spheres divides into two, and the ovum thus becomes constituted 
of twelve spheres, eight outer and four inner. The outer spheres 
have now become markedly smaller than the inner. 

The four inner spheres next divide giving rise, together with 
the eight outer spheres, to sixteen spheres in all ; which are 
nearly uniform in size. Of the eight inner spheres four soon 
pass to the centre, while the eight now superficial outer spheres 
form a kind of cup partially enclosing the inner spheres. The 
outer spheres now divide in their turn, giving rise to sixteen 

1 The cells spoken of as the outer layer correspond to Van Beneden's 
epiblast, whilst those cells spoken of as the inner correspond to his 
primitive hypoblast. 



314 THE MAMMALIAN EMBEYO. [CHAP. 

spheres which largely enclose the inner spheres. The segmenta- 
tion of both outer and inner spheres continues, and in the course 
of it the outer spheres spread further and further over the inner, 
so that at the close of segmentation the inner spheres constitute a 
central solid mass almost entirely surrounded by the outer 
spheres. In a small circular area however the inner mass of 
spheres remain for some time exposed at the surface (Fig. 95 A). 

The blastodennic vesicle. After its segmentation 
the ovum passes into the uterus. The outer cells soon 
grow over the blastopore and thus form a complete 
superficial layer. A series of changes next take place 
which result in the formation of what has been called 
the blastodermic vesicle. 

These changes commence with the appearance of a 
narrow cavity between the outer and inner layers, which 
extends so as completely to separate them except in the 
region adjoining the original site of the blastopore (Fig. 
95 B) 1 . The cavity so formed rapidly enlarges, and 
with it the ovum also ; so that this soon takes the form 
of a thin walled vesicle with a large central cavity. 
This vesicle is the blastodermic vesicle. The greater 
part of its walls are formed of a single row of flattened 
outer layer cells; while the inner mass of cells forms 
a small lens-shaped mass attached to the inner side of 
the outer layer (Fig. 96). 

Although by this stage, which occurs in the rabbit 
between seventy and ninety hours after impregnation, 
the blastodermic vesicle has by no means attained its 
greatest dimensions, it has nevertheless grown from 

1 Van Beheden regards it as probable that the blastopore is 
situated somewhat excentrically in relation to the area of attachment 
of the inner mass to the outer layer. 



X.] 



BLASTODERMIC VESICLE. 



315 



about O09 mm. the size of the ovum at the close 
segmentation to about 0*28 in diameter. It is en- 
closed by the zona radiata and the albuminous layer 



FIG. 




BABBIT'S OVUM BETWEEN 70 90 HOURS AFTER IMPREGNATION. 
(After E. van Beneden.) 

bv. cavity of blastodermic vesicle (yolk-sac) ; ep. outer layer ; 
hy. inner mass ; Zp. albuminous envelope. 



around it. The blastodermic vesicle continues to 
enlarge rapidly, and during the process the inner mass 
undergoes important changes. It spreads out on the 
inner side of the outer layer and at the same time loses 
its lens-like form and becomes flattened. The central 



316 THE MAMMALIAN EMBRYO. [CHAP. 

part of it remains however thicker, and is constituted 
of two rows of cells, while the peripheral part, the outer 
boundary of which is irregular, is formed of an imperfect 
layer of amoeboid cells which continually spread further 
and further beneath the outer layer. The central thick- 
ening of the inner layer forms an opaque circular spot 
on the blastoderm, which constitutes the commencement 
of the embryonic area. 

The formation of the layers. The history of the 
stages immediately following, from about the com- 
mencement of the fifth day to the seventh day, when a 
primitive streak makes its appearance, is not perfectly 
understood, and has been interpreted very differently by 
various observers. The following account must there- 
fore be considered as a tentative one. 

About five days after impregnation the cells of the 
inner mass in the embryonic area become divided into 
two distinct strata, an upper stratum of rounded cells 
adjoining the flattened outer layer and a lower stratum 
of flattened cells. This lower stratum is the true hypo- 
blast (Fig. 97). At the edge of the embryonic area the 
hypoblast is continuous with a peripheral ring of the 
amosboid cells of the earlier stage, which now form, 
except at the edge of the ring, a continuous layer of 
flattened cells in contact with the outer layer. During 
the sixth day the middle layer becomes fused with the 
outer layer, and gives rise to a layer of cells which are 
columnar and are arranged in the rabbit in a single 
row (Fig. 98). They form together the true epiblast of 
the embryonic area. 

At this stage therefore the embryonic area, which is 
circular, is formed throughout of two single layers of 



X.] 



FORMATION OF THE LAYERS. 



317 



cells, a columnar epiblast and a layer of flattened hypo- 
blast. 

Fm. 97. 




SECTION THROUGH THE NEARLY CIRCULAR EMBRYONIC AREA OF 

A RABBIT OVUM OF Six DAYS. 

(From Allen Thomson, after E. van Beneden.) 

ect. upper layer ; mes. middle layer ; ent. true hypoblast. 

FIG. 98. 




SECTION THROUGH THE BLASTODERM OF A RABBIT ON THE 
SEVENTH DAY : TAKEN IN FRONT OF THE PRIMITIVE 
STREAK. 

Half of the area is represented. 

Towards the end of the sixth day the embryonic 
area of the rabbit, which has hitherto been round, be- 
comes oval. 

A diagrammatic view of the whole blastodermic 
vesicle at about the beginning of the seventh day is 
given in Fig. 99. The embryonic area is represented in 
white. The line ge in B shows the extension of the 
hypoblast round the inside of the vesicle. The bias- 



318 



THE MAMMALIAN EMBRYO. 

FIG. 99. 
A. 



[CHAP. 




VIEWS OF THE JBLASTODERMIC VESICLE OF A KABBIT ON THE 

SEVENTH DAY WITHOUT THE ZONA. A. from above, B. 
from the side. (From Kolliker.) 

ag. embryonic area ; ge. boundary of the hypoblast. 



X.] PRIMITIVE STREAK. 319 

todermic vesicle is therefore formed of three areas, 
(1) the embryonic area with two layers, a columnar 
epiblast and flat hypoblast; (2) the region around the 
embryonic area where the walls of the vesicle are formed 
of flattened epiblast 1 and of hypoblast ; (3) the area 
beyond this again where the vesicle is formed of flat- 
tened epiblast 1 only. 

The changes which next take place begin with the 
formation of a primitive streak, homologous with, and in 
most respects similar to, the primitive streak in Birds. 

FIG. 100. 




EMBRYONIC AREA OF AN EIGHT DAYS' RABBIT. 

(After Kolliker.) 
arg. embryonic area ; pr. primitive streak. 

The formation of the streak is preceded by that of a 
dark spot near the middle of the blastoderm, forming 
the nodal point of Hensen. This spot subsequently 
constitutes the front end of the primitive streak. 

Early on the seventh day the embryonic area be- 
comes pyriform, and at its posterior and narrower end 

1 The epiblast of the blastodermic vesicle beyond the embryonic 
area is formed of the outer layer only. 



320 THE MAMMALIAN EMBRYO. [CHAP. 

the primitive streak makes its appearance ; it is due to 
a proliferation of rounded cells from the epiblast. 
FIG. 101. 

p.r 




SECTION THROUGH AN OVAL BLASTODERM or A RABBIT ON 
THE SEVENTH DAY. THE LENGTH OF THE AREA WAS 
ABOUT 1'2 MM. AND ITS BREADTH ABOUT '86 MM. 

Through the front part of the primitive streak ; ep. epiblast ; 
m. mesoblast ; hy. hypoblast ; pr. primitive streak. 

These cells give rise to a part of the mesoblastic 
layer of the embryo, and may be termed from their 
origin the primitive streak mesoblast. 

During the seventh day the primitive streak be- 
comes a more pronounced structure (Fig. 101), the 
mesoblast in its neighbourhood increases in quantity, 
while an axial groove (Fig. 100) the primitive groove 
is formed on its upper surface. 

The formation of the medullary groove. In the 
part of the embryonic area in front of the primitive 
streak there arise during the eighth day two folds 
bounding a shallow median groove, which meet in front, 
but diverge behind, and enclose between them the 
foremost end of the primitive streak (Fig. 103). These 
folds are the medullary folds and they constitute the 
first definite traces of the embryo. The medullary plate 
bounded by them rapidly grows in length, the primitive 
streak always remaining at its hinder end. While the 



X.] 



THE MESOBLAST. 

FIG. 102. 
A. 



321 




Two TRANSVERSE SUCTIONS THROUGH THE EMBRYONIC AREA 
OF AN EMBRYO RABBIT OF SEVEN DAYS. 

The embryo has nearly the appearance represented in Fig. 100. 

A. is taken through the anterior part of the embryonic area. 
It represents about half the breadth of the area, and there is no 
trace of a medullary groove or of the mesoblast. 

B. is taken through the posterior part of the primitive 
streak. 

ep. epiblast ; hy. hypoblast. 

lateral epiblast is formed of several rows of cells, that of 
the medullary plate is at first formed of but a single 
row (Fig. 104, mg). 

The mesoblast and notochord. The mesoblast in 
mammalia has, as in the chick, a double origin, and the 
details of its development appear to resemble essentially 
those in the chick. It arises (1) from the epiblast of 
the primitive streak ; this has been already described ; 
(2) from the primitive hypoblast in front and at the 
sides of the primitive streak. The latter is known as 
hypoblastic mesoblast, and as in the chick appears to 
originate as two lateral plates split off from the primi- 
tive hypoblast. These two plates are at first continuous 
F. &B. 21 



322 



THE MAMMALIAN EMBRYO. [CHAP. 

Fia. 103. 




EMBRYONIC AREA OF A SEVEN DAYS' EMBRYO RABBIT. 
(From Kolliker.) 

o. place of future area vasculosa ; rf. medullary groove ; pr. pri- 
mitive streak ; ag. embryonic area. 

In the region o. a layer of mesoblast has already grown ; there 
are however as yet no signs of blood-vessels in it. 

This mesoblast is derived from the mesoblast of the primitive 
streak (Kolliker). 

in the axial line with the primitive hypoblast. When 
the medullary groove is formed the lateral bands of 
raesoblast become separate from the axial hypoblast and 
give rise to two independent lateral plates of mesoblast 



X.] THE PRIMITIVE STREAK. 323 

(Fig. 104). The axial band of hypoblast eventually 
oives rise to the notochord. 



FIG. 104. 




TRANSVERSE SECTION THROUGH AN EMBRYO RABBIT OF EIGHT 

DAYS. 

efj. epiblast ; me. mesoblast ; Jiy. hypoblast ; mg. medullary 
groove. 

The mesoblastic elements from these two sources, 
though at first characterised by the difference in the 
appearance of their cells (Fig. 102, B), those of the 
primitive streak mesoblast being more rounded, soon 
become blended and indistinguishable from one another; 
so that it is difficult to say to what parts of the fully 
formed mesoblast they severally contribute. 

In tracing the changes which take place in the rela- 
tions of the layers, while passing from the region of the 
embryo to that of the primitive streak, it will be con- 
venient to follow the account given by Schafer for the 
guinea-pig, which on this point is far fuller and more 
satisfactory than that of other observers. In doing so 
we shall leave out of consideration the fact that the 
layers in the guinea-pig are inverted. Fig. 105 repre- 
sents a series of sections through this part in the guinea- 
pig. The anterior section (D) passes through the medul- 
lary groove near its hinder end. The commencement of 
the primitive streak is marked by a slight prominence on 
the floor of the medullary groove between the two diverg- 

212 



324 



THE MAMMALIAN EMBRYO. 



[CHAP. 



ing medullary folds (Fig. 105 C, ae). Where this promi- 
nence becomes first apparent the epiblast and hypoblast 




A SERIES OF TRANSVERSE SECTIONS THROUGH THE JUNCTION 
OF THE PRIMITIVE STREAK AND MEDULLARY GROOVE OF 
A YOUNG GUINEA-PIG. (After Schafer.) 

A. is the posterior section. 

e. epiblast ; m. mesoblast ; h. hypoblast ; ae. axial epiblast of 
the primitive streak ; ah. axial hypoblast attached in B. and 
C. to the epiblast at the rudimentary blastopore ; ng. me- 
dullary groove ; /. rudimentary blastopore. 



X.] THE NOTOCHORD. 325 

are united together. The mesoblast plates at the two 
sides remain in the meantime quite free. Slightly 
further back, but before the primitive groove is reached, 
the epiblast and hypoblast are connected together by a 
cord of cells (Fig. 105 B,/), which in the section next 
following becomes detached from the hypoblast and 
forms a solid keel projecting from the epiblast. In the 
following section the hitherto independent mesoblast 
plates become united with this keel (Fig. 105 A) ; and 
in the posterior sections, through the part of the primi- 
tive streak with the primitive groove, the epiblast and 
mesoblast continue to be united in the axial line, but 
the hypoblast remains distinct. These peculiar relations 
may shortly be described by saying that in the axial 
line the hypoblast becomes united with the epiblast at 
the posterior end of the embryo; and that the cells 
which connect the hypoblast and epiblast are posteriorly 
continuous with the fused epiblast and mesoblast of 
the primitive streak, the hypoblast in the region of the 
primitive streak having become distinct from the other 
layers. 

The notochord. The thickened axial portion of the 
hypoblast in the region of the embryo becomes sepa- 
rated, as we have already pointed out, from the lateral 
parts as the notochord. 

Very shortly after the formation of the notochord, 
the hypoblast grows in from the two sides, and becomes 
quite continuous across the middle line. The formation 
of the notochord takes place from before backwards; 
and at the hinder end of the embryo it is continued 
into the mass of cells which forms the axis of the primi- 
tive streak, becoming therefore at this point continuous 



326 THE MAMMALIAN EMBRYO. [CHAP. 

with the epiblast. The notochord in fact behaves exactly 
as did the axial hypoblast in the earlier stage. 

The peculiar relations just mentioned are precisely similar to 
those we have already described in the chick (p. 60). They 
receive their explanation by comparison with the lower types. 

The cells which form the junction between the epiblast and 
the axial hypoblast constitute in the lower types the front wall of 
a passage perforating the blastoderm and leading from the ex- 
terior into the alimentary canal. This passage is the vertebrate 
blastopore. 

In the chick we have seen (p. 72) this passage is present at a 
certain stage of development as the neurenteric canal ; and in the 
duck at a still earlier stage. It is also present at an early stage 
in the mole. 

The presence of this blastopore renders it clear that the blas- 
topore discovered by Ed. van Beneden cannot have the meaning 
he assigned to it in comparing it with the blastopore of the 



To recapitulate. At the stage we have now reached 
the three layers are definitely established. 

The epiblast is derived partly from the outer layer 
of segmentation spheres and partly from the larger pro- 
portion of those segmentation spheres which constitute 
the inner mass. The hypoblast arises from the few 
remaining cells of the inner mass ; while the mesoblast 
has its origin partially from the epiblast of the primitive 
streak and partially from the hypoblast cells anterior to 
the primitive streak. 

During the period in which these changes have been taking 
place, the rudiments of a vascular area become formed, and while 
as Kolliker has shewn, the mesoblast of this portion is to some 
extent derived from the mesoblast of the primitive streak, it is 
possible that a portion of it owes its origin to hypoblastic meso- 
blast. 



X.] THE MEDULLARY PLATE. 327 

General growth of the embryo. We have seen 
that the blastodermic vesicle becomes divided at an 
early stage of development into an embryonic area, and 
a non-embryonic portion. The embryonic area gives 
rise to the whole of the body of the embryo, while the 
non-embryonic part forms an appendage known as the 
umbilical vesicle, which becomes gradually folded off 
from the embryo, and has precisely the relations of the 
yolk-sac of the chick. It is almost certain that the 
Mammalia are descended from ancestors, the embryos 
of which had large yolk-sacs, but that the yolk has 
become reduced in quantity owing to the nutriment 
received from the wall of the uterus taking the place 
of that originally supplied by the yolk. A rudiment of 
the yolk-sac being thus retained in the umbilical vesi- 
cle, this structure may be called indifferently umbilical 
vesicle or yolk-sac. 

The yolk which fills the yolk-sac in Birds is re- 
placed in Mammals by a coagulable fluid; while the 
gradual extension of the hypoblast round the wall of 
the blastodermic vesicle, which has already been de- 
scribed, is of the same nature as the growth of the hy- 
poblast round the yolk-sac in Birds. 

The whole embryonic area would seem to be em- 
ployed in the formation of the body of the embryo. Its 
long axis has no very definite relation to that of the 
blastodermic vesicle. The first external trace of the 
embryo to appear is the medullary plate, bounded by 
the medullary folds, and occupying at first the anterior 
half of the -embryonic area (Fig. 103). The two me- 
dullary folds diverge behind and enclose the front end 
of the primitive streak. As the embryo elongates the 



328 THE MAMMALIAN EMBRYO. [CHAP. 

medullary folds nearly meet behind and so cut off the 
front portion of the primitive streak, which then ap- 
pears as a projection in the hind end of the medullary 
groove. At the hind end of the medullary groove 
(mole) a deep pit perforates its floor and enters the 
mass of mesoblast cells lying below. The pit is a rudi- 
ment of the blastopore (described on p. 326) which has 
been enclosed by the medullary folds. 

Henceforward the general course of development is 
very similar to that in the chick and so will be only briefly 
described. The special features in the development of 
particular organs will be described later. In an embryo 
rabbit, eight days after impregnation, the medullary 
groove is about 1*80 mm. in length. At this stage a 
division may be clearly seen in the lateral plates of 
mesoblast into a vertebral zone adjoining the embryo 
and a more peripheral lateral zone ; and in the verte- 
bra] zone indications of two somites, about 0'37 mm. 
from the hinder end of the embryo, become apparent. 
The foremost of these somites marks the junction, or 
very nearly so, of the cephalic region and trunk. The 
small size of the latter as compared with the former is 
very striking, but is characteristic of Vertebrates gene- 
rally. The trunk gradually elongates relatively to the 
head, by the addition behind of fresh somites. The 
embryo has not yet begun to be folded off from the 
yolk-sac. 

In a slightly older embryo of nine days there appears 
(Hensen, Kolliker) round the embryonic area a delicate 
clear ring which is narrower in front than behind (Fig. 
106 A. ap). This ring is regarded by these authors as 
representing the peripheral part of the area pellucida of 



X.] THE CEKEBRAL VESICLES. 329 

Birds, which does not become converted into the body 
of the embryo. Outside the area pellucida, an area 
vasculosa has become very well defined. In the em- 
bryo itself (Fig. 106 A) the disproportion between head 
and trunk is less marked than before; the medullary 
plate dilates anteriorly to form a spatula-shaped ce- 
phalic enlargement; and three or four somites are 
established. In the lateral parts of the mesoblast of 
the head there may be seen on each side a tube-like 
structure (hz). Each of these is part of the heart, which 
arises as two independent tubes. The remains of the 
primitive streak (pr) are still present behind the me- 
dullary groove. 

In somewhat older embryos (Fig. 106 B) with about 
eight somites, in which the trunk considerably exceeds 
the head in length, the first distinct traces of the 
folding off of the head end of the embryo become ap- 
parent, and somewhat later a fold also appears at the 
hind end. In the formation of the hind end of the 
embryo the primitive streak gives rise to a tail swelling 
and to part of the ventral wall of the post-anal gut. In 
the region of the head the rudiments of the heart (h) 
are far more definite. The medullary groove is still 
open for its whole length, but in the head it exhibits a 
series of well-marked dilatations. The foremost of 
these (vh) is the rudiment of the fore-brain from the 
sides of which there project the two optic vesicles (all) ; 
the next is the mid-brain (mK) and the last is the hind-- 
brain (hh), which is again divided into smaller lobes by 
successive constrictions. The medullary groove behind 
the region of the somites dilates into an embryonic 
sinus rhomboidalis like that of the bird. Traces of the 



330 



A. 



THE MAMMALIAN EMBRYO. 

FIG. 106. 

B. 



[CHAP. 




EMBRYO RABBITS OF ABOUT NINE DAYS FROM THE DORSAL SIDE. 

(From Kolliker.] 
A. magnified 22 times, and B. 21 times. 

ap. area pellucida ; rf. medullary groove ; h f . medullary plate in 
the region of the future fore-brain ; h". medullary plate in 
the region of the future mid-brain ; vh. fore- brain ; ab. optic 
vesicle ; mh. mid-brain ; lili. and h'". hind-brain ; uw. meso- 
blastic somite ; stz. vertebral zone ; pz. lateral zone ; liz. and 
h. heart ; ph. pericardial section of body-cavity ; vo. vitelline 
vein ; af. amnion fold. 



X.] GENERAL DEVELOPMENT. 331 

amnion (of) are now apparent both in front of and 
behind the embryo. 

The structure of the head and the formation of the 
heart at this age are illustrated in Fig. 107. The 
widely open medullary groove (rf) is shewn in the 
centre. Below it the hypoblast is thickened to form 
the notochord dd' ; and at the sides are seen the two 
tubes, which, on the folding-in of the fore-gut, give rise 
to the unpaired heart 1 . Each of these is formed of 
an outer muscular tube of splanchnic mesoblast (ahh), 
not quite closed towards the hypoblast, and an inner 
epithelioid layer (ihh), and is placed in a special section 
of the body cavity (ph), which afterwards forms the 
pericardial cavity. 

Before the ninth day is completed great external 
changes are usually effected. The medullary groove 
becomes closed for its whole length with the exception 
of a small posterior portion. The closure commences, 
as in Birds, in the region of the mid-brain. Anteriorly 
the folding-off of the embryo proceeds so far that the 
head becomes quite free, and a considerable portion of 
the throat, ending blindly in front, becomes established. 
In the course of this folding the, at first widely sepa- 
rated, halves of the heart are brought together, coalesce 
on the ventral side of the throat, and so give rise to a 
median undivided heart. The fold at the tail end of 
the embryo progresses considerably, and during its ad- 
vance the allantois is formed in the same way as in 
Birds. The somites increase in number to about twelve. 
The amniotic folds nearly meet above the embryo. 

1 The details of the development of the heart are described below 
(ch. xii.). 



832 



A. 



THE MAMMALIAN EMBRYO, [CHAP. 

FIG. 107. 




TRANSVERSE SECTION THROUGH THE HEAD OF A KABBIT OF 
THE SAME AGE AS FIG. 106 B. (From Kolliker.) 

B. is a more highly magnified representation of part of A. 

rf. medullary groove ; mp. medullary plate ; rw. medullary fold ; 
h. epiblast ; dd. hypoblast ; dd'. notochordal thickening of 
hypoblast ; sp. undivided mesoblast ; hp. somatic mesoblast ; 



X.] THE CRANIAL FLEXURE. 333 

dfjj. splanchnic mesoblast; ph. pericardial section of body- 
cavity ; ahh. muscular wall of heart ; ihh. epithelioid layer of 
heart ; mes. lateral undivided mesoblast ; sw. fold of hypo- 
blast which will form the ventral wall of the pharynx ; sr. 
commencing throat. 

The later stages in the development proceed in the 
main in the same manner as in the Bird. The cranial 
flexure soon becomes very marked, the mid-brain form- 
ing the end of the long axis of the embryo (Fig. 108). 
The sense organs have the usual development. Under 
the fore -brain appears an epiblastic involution giving 

FIG. 108. 




ADVANCED EMBRYO OF A RABBIT (ABOUT TWELVE DAYS) 1 . 

mb. mid-brain ; ih. thalamencephalon ; ce. cerebral hemisphere ; 
op. eye ; iv.v. fourth ventricle ; moc. maxillary process ; md. 
mandibular arch ; Jiy. hyoid arch ; fl. fore-limb ; hi. hind- 
limb ; urn. umbilical stalk. 

1 .This figure was drawn by Mr Weldon. 



334 THE MAMMALIAN EMBRYO. [CHAP. 

rise both to the mouth and to the pituitary body. Be- 
hind the mouth are three well marked pairs of visceral 
arches. The first of these is the mandibular arch 
(Fig. 108 md) y which meets its fellow in the middle 
line, and forms the posterior boundary of the mouth. 
It sends forward on each side a superior maxillary pro- 
cess (mx) which partially forms the anterior margin of 
the mouth. Behind the mandibular arch are present a 
well-developed hyoid (hy) and a first branchial arch 
(not shewn in Fig. 108). There are four clefts, as in 
the chick, but the fourth is not bounded behind by a 
definite arch. Only the first of these clefts persists as 
the tympanic cavity and Eustachian tube. 

At the time when the cranial flexure appears, the 
body also develops a sharp flexure immediately behind 
the head, which is thus bent forwards upon the pos- 
terior straight part of the body (Fig. 108). The amount 
of this flexure varies somewhat in different forms. It 
is very marked in the dog (Bischoff ). At a later period, 
and in some species even before the stage figured, the 
tail end of the body also becomes bent (Fig. 108), so 
that the whole dorsal side assumes a convex curvature, 
and the head and tail become closely approximated. In 
most cases the embryo, on the development of the tail, 
assumes a more or less definite spiral curvature (Fig. 
108). With the more complete development of the 
lower wall of the body the ventral flexure partially dis- 
appears, but remains more or less persistent till near 
the close of intra-uterine life. The limbs are formed as 
simple buds in the same manner as in Birds. The buds 
of the hind-limbs are directed somewhat forwards, and 
those of the fore-limb backwards. 



X.] THE HUMAN EMBRYO. 335 

The human embryo. Our knowledge as to the 
early development of the human embryo is in an un- 
satisfactory state. The positive facts we know are com- 
paratively few, and it is not possible to construct from 
them a history of the development which is capable of 
satisfactory comparison with that in other forms, unless 
all the early embryos known are to be regarded as 
abnormal. The most remarkable feature in the develop- 
ment, which was first clearly brought to light by Allen 
Thomson in 1839, is the very early appearance of 
branched villi. In the last few years several ova, even 
younger than those described by Allen Thomson, have 
been met with, which exhibit this peculiarity. 

The best preserved of these ova is one described by 
Reichert 1 . This ovum, though probably not more than 
thirteen days old, was completely enclosed by a decidua 
reflexa. It had (Fig. 109 A and B) a flattened oval 
form, measuring in its two diameters 5 '5 mm. and 
3*5 mm. The edge was covered with branched villi, 
while in the centre of each of the flattened surfaces 
there was a spot free from villi. On the surface ad- 
joining the uterine wall was a darker area (e) formed of 
two layers of cells. Nothing certain has been made out 
about the structure of ova of this age. 

The villi, which at first leave the flattened poles 
free, seem soon to extend first over one of the flat sides 
and finally over the whole ovum (Fig. 109 C). 

Unless the two-layered region of Reichert's ovum is 
the embryonic area, nothing which can clearly be 
identified as an embryo has been detected in these 

1 Abhandlungen der Konigl. Akad. d. Wiss. zu Berlin, 1873. 



336 



THE MAMMALIAN EMBRYO. 



[CHAP. 




THE HUMAN OVA DURING EARLY STAGES OP DEVELOPMENT. 
(From Quain's Anatomy.) 

A. and B. Front and side view of an ovum figured by Keichert, 

supposed to be about thirteen days. e. embryonic area. 
C. An ovum of about four or five weeks shewing the general 

structure of the ovum before the formation of the placenta. 

Part of the wall of the ovum is removed to shew the embryo 

in situ. (After Allen Thomson.) 

early ova. In an ovum described by Breus, and in one 
described long ago by Wharton-Jones, a mass found in 
the interior of the ovum may perhaps be interpreted 
(His) as the remains of the yolk. It is, however, very 
probable that all the early ova so far obtained are 
more or less pathological. 

The youngest ovum with a distinct embryo is one 
described by His. This ovum, which is diagrammati- 
cally represented in Fig. Ill in longitudinal section, 
had the form of an oval vesicle completely covered by 
villi, being about 8*5 mm. and 5*5 mm. in its two 
diameters, and flatter on one side than on the other. 
An embryo with a yolk-sac was attached to the inner 
side of the flatter wall of the vesicle by a stalk, which 
must be regarded as the allantoic stalk; the embryo 



X.] 



THE HUMAN EMBRYO. 
FIG. 110. 



337 



ch- 




THREE EARLY HUMAN EMBRYOS. (Copied from His.) 

A. Side view of an early embryo described by His. 

B. Embryo of about 12 14 days described by Allen Thom- 
son. 

C. Young embryo described by His. 

am. amnion ; md. medullary groove ; um. umbilical vesicle ; 
ck. chorion, to which the embryo is attached by a stalk. 

and yolk-sac filled up but a very small part of the 
whole cavity of the vesicle. 

The embryo, which was probably not quite normal 
(Fig. 110 A), was very imperfectly developed; a me- 
dullary plate was hardly indicated, and, though the 
mesoblast was unsegmented, the head fold, separating 
the embryo from the yolk-sac (um), was already in- 
F. & B. 22 



338 THE MAMMALIAN EMBRYO. [CHAP. 




DIAGRAMMATIC LONGITUDINAL SECTION OF THE OVUM TO 

WHICH THE EMBRYO (FiG. 110 A.) BELONGED. (After His.) 

am. amnion ; Nb. umbilical vesicle. 

dicated. The amnion (am) was completely formed, and 
vitelline vessels had made their appearance. 

Two embryos described by Allen Thomson are but 
slightly older than the above embryo of His. Both of 
them probably belong to the first fortnight of preg- 
nancy. In both cases the embryo was more or less 
folded off from the yolk-sac, and in one of them the 
medullary groove was still widely open, except in the 
region of the neck (Fig. 110 B). The allantoic stalk, if 
present, was not clearly made out, and the condition of 
the amnion was also not fully studied. The smaller of 
the two ova was just 6 mm. in its largest diameter, and 
was nearly completely covered with simple villi, more 
developed on one side than on the other. 

In a somewhat later period, about the stage of a 
chick at the end of the second day, the medullary folds 
are completely closed, the region of the brain already 
marked, and the cranial flexure commencing. The 
mesoblast is divided up into numerous somites, and the 
mandibular and first two branchial arches are indicated. 



X.] 



THE HUMAN EMBRYO. 



339 



The embryo is still but incompletely folded off from 
the yolk-sac below. 

In a still older stage the cranial flexure becomes 
still more pronounced, placing the mid-brain at the end 
of the long axis of the body. The body also begins to 
be ventrally curved (Fig. 110 C). 

Externally human embryos at this age are charac- 
terized by the small size of the anterior end of the 
head. 

The flexure goes on gradually increasing, and in the 
third week of pregnancy in embryos of about 4 mm. the 
limbs make their appearance. 

The embryo at this stage (Fig. 112), which is about 

FIG. 112. 




Two VIEWS OP A HUMAN EMBRYO OF BETWEEN THE THIRD 
AND FOURTH WEEK. 

A. Side view. (From Kolliker ; after Allen Thomson.) a. 
amnion ; b. umbilical vesicle ; c. mandibular arch ; e. hyoid 
arch; /. commencing anterior limb; g. primitive auditory 
vesicle ; h. eye ; i. heart. 

B. Dorsal view to shew the attachment of the dilated allantoic 
stalk to the chorion. (From a sketch by Allen Thomson.) 
am. amnion ; all. allantois ; ys. yolk-sac. 

222 



340 



THE MAMMALIAN EMBRYO. 



[CHAP. 



equivalent to that of a chick on the fourth day, re- 
sembles in almost every respect the normal embryos of 
the Amniota. The cranial flexure is as pronounced as 
usual, and the cerebral region has now fully the normal 
size. The whole body soon becomes flexed ventrally, 
and also somewhat spirally. The yolk-sac (B ; ys) forms a 
small spherical appendage with a long wide stalk, and 
the embryo is attached by an allantoic stalk with a 
slight swelling, probably indicating the presence of a 
small hypoblastic diverticulum, to the inner face of the 
chorion. 

A detailed history of the further development of 
the human embryo does not fall within the province of 

FIG. 113. 




FIGURES SHEWING THE EARLY CHANGES IN THE FORM OF THE 
HUMAN HEAD. (From QM&IU'S Anatomy.) 

A. Head of an embryo of about four weeks. (After 

Allen Thomson.) 

B. Head of an embryo of about six weeks. (After Ecker.) 

C. Head of an embryo of about nine weeks. 

1. mandibular arch ; 1'. persistent part of hyomandibular cleft ; 
a. auditory vesicle. 



INVERSION OF THE LAYERS. 



341 



x.] 

this work; while the later changes in the embryonic 
membranes will be dealt with in the next chapter. For 
the changes which take place on the formation of the 
face we may refer the reader to Fig. 113. For a full dis- 
cussion as to the relation between the human embryos 
just described and those of other Mammals, we refer the 
reader to the Comp. Embryology, Vol. II. p. 224 et seq. 
The guinea pig, rat and mouse present a pe- 
culiar method of development, the details of which are 
not entirely understood, and we do not propose to 
examine them here. Suffice it to say that the mode of 
development gives rise to the so-called inversion of the 
layers; so called because the outer layer of the em- 
bryonic vesicle appeared to the older observers to be 
formed of hypoblast and the embryonic epiblast to be 
enclosed within. 



CHAPTER XL 

EMBRYONIC MEMBRANES AND YOLK-SAC. 

IN the Mammalia the early stages in the develop- 
ment of the embryonic membranes are nearly the same 
as in Aves ; but during the later stages the allantois 
enters into peculiar relations with the uterine walls, 
and the two, together with the interposed portion of 
the sub zonal membrane or false amnion (the nature of 
which will be presently described), give rise to a very 
characteristic Mammalian organ the placenta into 
the structure of which it will be necessary to enter 
at some length. The embryonic membranes vary so 
considerably in the different forms that it will be ad- 
vantageous to commence with a description of their 
development in an ideal case. 

We may commence with a blastodermic vesicle closely 
invested by the delicate remnant of the zona radiata at 
the stage in which the medullary groove is already 
established. Around the embryonic area a layer of 
mesoblast would have extended for a certain distance ; 
so as to give rise to an area vasculosa, in which how- 
ever the blood-vessels would not have become definitely 



CHAP. XI.] MEMBRANES OF RABBIT. 343 

established. Such a vesicle is represented diagram- 
matically in Fig. 114, I. Somewhat later the embryo 
begins to be folded off first in front and then behind 
(Fig. 114, 2). These folds result in a constriction sepa- 
rating the embryo and the yolk-sac (ds), or as it is 
called in Mammalian embryology, the umbilical vesicle. 
The splitting of the mesoblast into a splanchnic and a 
somatic layer has taken place, and at the front and 
hind end of the embryo a fold (ks) of the somatic meso- 
blast and epiblast begins to rise up and grow over the 
head and tail of the embryo. These two folds form the 
commencement of the amnion. The head and tail folds 
of the amnion are continued round the two sides of the 
embryo till they meet and unite into a continuous fold. 
This fold grows gradually upwards, but before it has 
completely enveloped the embryo the blood-vessels of 
the area vasculosa become fully developed. They are 
arranged in a manner not very different from that in 
the chick. 

The following is a brief account of their arrange- 
ment in the rabbit : 

The outer boundary of the area, which is continually extend- 
ing further and further round the umbilical vesicle, is marked by 
a venous sinus terminalis (Fig. 114, st). The area is not, as in 
the chick, a nearly complete circle, but is in front divided by a 
deep indentation extending inwards to the level of the heart. In 
consequence of this indentation the sinus terminalis ends in 
front in two branches, which bend inwards and fall directly into 
the main vitelliue veins. The blood is brought from the dorsal 
aortse by a series of lateral vitelline arteries, and not by a single 
pair as in the chick. These arteries break up into a more deeply 
situated arterial network, from which the blood is continued 
partly into the sinus terminalis, and partly into a superficial venous 



344 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. 
FIG. 1U. 




ch 



XI.] EMBRYONIC MEMBRANES. 345 



FIVE DIAGRAMMATIC FIGURES ILLUSTRATING THE FORMATION 
OF THE FOETAL MEMBRANES OF A MAMMAL. (From Kolli- 
ker.) 

In 1, 2, 3, 4 the embryo is represented in longitudinal section. 

1. Ovurn with zona pellucicla, blastodermic vesicle, and 
embryonic area. 

2. Ovum with commencing formation of umbilical vesicle 
and amnion. 

3. Ovum with amnion about to close, and commencing 
allantois. 

4. Ovum with villous subzonal membrane, larger allantois, 
and mouth and anus. 

5. Ovum in which the mesoblast of the allantois has ex- 
tended round the inner surface of the subzonal membrane and 
united with it to form the chorion. The cavity of the allantois 
is aborted. This fig. is a diagram of an early human ovum. 

d. zona radiata ; d and sz. processes of zona ; sh. subzonal mem- 
brane, outer fold of amnion, false amnion ; ch. chorion ; ch. z. 
chorionic villi ; am. amnion ; ks. head-fold of amnion ; ss. tail- 
fold of amnion ; a. epiblast of embryo ; a. epiblast of non-em- 
bryonic part of the blastodermic vesicle ; m. embryonic meso- 
blast ; m'. non-embryonic mesoblast ; df. area vasculosa ; st. 
sinus terminalis; dd. embryonic hypoblast; i. non-embryo- 
nic hypoblast ; kh. cavity of blastodermic vesicle, the greater 
part of which becomes the cavity of umbilical vesicle ds. ; 
dg. stalk of umbilical vesicle ; al. allantois ; e. embryo ; r. 
space between chorion and amnion containing albuminous 
fluid ; vl. ventral body wall ; hh. pericardial cavity. 



346 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. 

network. The hinder end of the heart is continued into two 
vitelline veins, each of which divides into an anterior and a 
posterior branch. The anterior branch is a limb of the sinus 
terminalis, and the posterior and smaller branch is continued 
towards the hind part of the sinus, near which it ends. On its 
way it receives, on its outer side, numerous branches from the 
venous network. The venous network connects by its anasto- 
moses, the posterior branch of the vitelline vein and the sinus 
terminalis. 

Shortly after the establishment of the circulation of 
the yolk-sac the folds of the amnion meet and coalesce 
above the embryo (Fig. 114, 3 and 4, am). After this the 
inner or true amnion becomes severed from the outer 
or false amnion, though the two sometimes remain con- 
nected by a narrow stalk. The space between the true 
and false amnion is a continuation of the body cavity. 
The true amnion consists of a layer of epiblastic epi- 
thelium and generally also of somatic mesoblast, while 
the false amnion consists as a rule of epiblast only; 
though it is possible that in some cases (the rabbit ?) 
the mesoblast may be continued along its inner 
face. 

Before the two limbs of the amnion are completely 
severed the epiblast of the umbilical vesicle becomes sepa- 
rated from the subjacent mesoblast and hypoblast of the 
vesicle (Fig. 114, 3), and, together with the false am- 
nion (sh) with which it is continuous, forms a complete 
lining for the inner face of the zona radiata. The space 
between this membrane and the umbilical vesicle with 
the attached embryo is obviously continuous with the 
body cavity (vide Figs. 114, 4 and 115). To this mem- 
brane Turner has given the appropriate name of sub- 
zonal membrane : by Von Baer it was called the serous 



XI.] ATTACHMENT OF THE OVUM. 347 

envelope. It soon fuses with the zona radiata, or at 
any rate the zona ceases to be distinguishable. 

While the above changes have been taking place 
the whole blastodermic vesicle, still enclosed in the 
zona, has become attached to the walls of the uterus. 
In the case of the typical uterus with two tubular 
horns, the position of each embryo, when there are 
several, is marked by a swelling in the walls of the 
uterus, preparatory to the changes in the wall which 
take place on the formation of the placenta. In the 
region of each swelling the zona around the blasto- 
dermic vesicle is closely embraced in a ring-like fashion 
by the epithelium of the uterine wall. The whole 
vesicle assumes an oval form, and it lies in the uterus 
with its two ends free. The embryonic area is placed 
close to the mesometric attachment of the uterus. In 
many cases peculiar processes or villi grow out from 
the ovum (Fig. 114, 4, sz) which fit into the folds of 
the uterine epithelium, The nature of these processes 
requires further elucidation, but in some instances 
they appear to proceed from the zona (rabbit) and in 
other instances from the subzonal membrane (dog). 
In any case the attachment between the blastodermic 
vesicle and the uterine wall becomes so close at the 
time when the body of the embryo is first formed out 
of the embryonic area, that it is hardly possible to 
separate them without laceration ; and at this period 
from the 8th to the 9th day in the rabbit it requires 
the greatest care to remove the ovum from the uterus 
without injury. It will be understood of course that 
the attachment above described is at first purely super- 
ficial and not vascular. 



348 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. 

During the changes above described as taking place 
in the amnion, the allantois grows out from the hind- 
gut as a vesicle lined by hypoblast, but covered ex- 
ternally by a layer of splanchnic mesoblast (Fig. 114, 3 
and 4, at) 1 . It soon becomes a flat sac, projecting into 
the now largely developed space between the subzonal 
membrane and the amnion, on the dorsal side of the 
embryo (Fig. 115, ALC}. In some cases it extends so 
as to cover the whole inner surface of the subzonal 
membrane ; in other cases again its extension is much 
more limited. Its lumen may be retained or may be- 
come nearly or wholly aborted. A fusion takes place 
between the subzonal membrane and the adjoining 
mesoblastic wall of the allantois, and the two together 
give rise to a secondary membrane round the ovum 
known as the chorion. Since however the allantois 
does not always come in contact with the whole inner 
surface of the subzonal membrane the term chorion is 
apt to be somewhat vague ; in the rabbit, for instance, 
a considerable part of the so-called chorion is formed 
by a fusion of the wall of the yolk-sac with the sub- 
zonal membrane (Fig. 116). The region of the chorion 
which gives rise to the placenta may in such cases be 
distinguished as the true chorion from the remaining 
part which will be called the false chorion. 

The mesoblast of the allantois, especially that part 
of it which assists in forming the chorion, becomes 
highly vascular ; the blood being brought to it by two 
allantoic arteries continued from the terminal bifur- 

1 The hypoblastic element in the allantois is sometimes very much 
reduced, so that the allantois maybe mainly formed of a vascular layer 
of mesoblast. 



XL] 



THE CHORION. 



FIG. 115. 



349 




DIAGRAM OF THE FCETAL MEMBRANES OP A MAMMAL. (From 
Turner.) 

Structures which either are or have been at an earlier period 
of development continuous with each other are represented by 
the same character of shading. 

pc. zona with villi ; sz. subzonal membrane ; E. epiblast of 
embryo ; am. amnion ; AC. amniotic cavity ; M. mesoblast 
of embryo ; H. hypoblast of embryo ; UV. umbilical vesicle ; 
al. allantois ; ALC. allantoic cavity. 

cation of the dorsal aorta, and returned to the body 
by one, or rarely two, allantoic veins, which join the 
vitelline veins from the yolk-sac. From the outer sur- 
face of the true chorion (Fig. 114, 5, ch. z, 116) villi grow 
out and fit into crypts or depressions which have in the 



350 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. 

meantime made their appearance in the walls of the 
uterus 1 . The villi of the chorion are covered by an 
epithelium derived from the subzonal membrane, and 
are provided with a connective-tissue core containing 
an artery and vein and a capillary plexus connecting 
them. In most cases they assume a more or less ar- 
borescent form, and have a distribution on the surface 
of the chorion varying characteristically in different 
species. The walls of the crypts into which the villi 
are fitted also become highly vascular, and a nutritive 
fluid passes from the maternal vessels of the placenta 
to the foetal vessels by a process of diffusion; while 
there is probably also a secretion by the epithelial 
lining of the walls of the crypts, which becomes ab- 
sorbed by the vessels of the fcetal villi. The above 
maternal and foetal structures constitute together the 
organ known as the placenta. The maternal portion 
consists essentially of the vascular crypts in the 
uterine walls, and the foetal portion of more or less 
arborescent villi of the true chorion fitting into these 
crypts. 

While the placenta is being developed the folding 
off of the embryo from the yolk-sac becomes more 
complete; and the yolk-sac remains connected with the 
ileal region of the intestine by a narrow stalk, the vi- 
telline duct (Fig. 114, 4 and 5 and Fig. 115), consisting 
of the same tissues as the yolk-sac, viz. hypoblast and 
splanchnic mesoblast. While the true splanchnic stalk 

1 These crypts have no connection with the openings of glands in 
the walls of the uterus. They are believed by Ercolani to be formed 
to a large extent by a regeneration of the lining tissue of the uterine 
walls. 



XL] THE PLACENTA. S51 

of the yolk-sac is becoming narrow, a somatic stalk 
connecting the amnion with the walls of the embryo is 
also formed, and closely envelopes the stalk both of the 
allantois and the yolk-sac. The somatic stalk together 
with its contents is known as the umbilical cord. The 
mesoblast of the somatopleuric layer of the cord de- 
velops into a kind of gelatinous tissue which cements 
together the whole of the contents. The allantoic ar- 

o 

teries in the cord wind in a spiral manner round the 
allantoic vein. The yolk-sac in many cases atrophies 
completely before the close of intra-uterine life, but in 
other cases it, like the other embryonic membranes, is 
not removed till birth. The intra-embryonic portion of 
the allantoic stalk gives rise to two structures, viz. to 
(1) the urinary bladder formed by a dilatation of its 
proximal extremity, and to (2) a cord known as the 
urachus connecting the bladder with the wall of the 
body at the umbilicus. The urachus, in cases where 
the cavity of the allantois persists till birth, remains as 
an open passage connecting the intra- and extra-em- 
bryonic parts of the allantois. In other cases it gradually 
closes, and becomes nearly solid before birth, though a 
delicate but interrupted lumen would appear to persist 
in it. It eventually gives rise to the ligamentum vesicae 
medium. 

At birth the foetal membranes, including the foetal 
portion of the placenta, are shed ; but in many forms 
the interlocking of the foetal villi with the uterine 
crypts is so close that the uterine mucous membrane is 
carried away with the foetal part of the placenta. It 
thus comes about that in some placentae the maternal 
and foetal parts simply separate from each other at birth, 



352 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. 

and that in others the two remain intimately locked 
together, and both are shed together as the after-birth. 
These two forms of placenta are distinguished as non- 
deciduate and deciduate, but no sharp line can be drawn 
between the two types. Moreover, a larger part of the 
uterine mucous membrane than that actually entering 
into the maternal part of the placenta is often shed in 
the deciduate Mammalia, and in the non-deciduate 
Mammalia it is probable that the mucous membrane 
(not including vascular parts) of the maternal placenta 
is either shed or absorbed. 



Comparative history of the Mammalian foetal 
membranes. 

Two groups of Mammalia the Monotremata and 
the Marsupialia are believed not to be provided with 
a true placenta. Nothing is known of the arrangement 
of the foetal membranes in the former group of animals 
(Monotremata). In the latter (Marsupialia) the yolk- 
sac is large and vascular, and is, according to Owen, 
attached to the subzonal membrane. The allantois on 
the other hand is but small, and is not attached to the 
subzonal membrane; it possesses however a vascular 
supply. 

Observations have hitherto been very limited with 
regard to the foetal membranes of this group of animals, 
but it appears highly probable that both the yolk-sac 
and the allantois receive nutriment from the walls of 
the uterus. 

All Mammalia other than the Monotremata and 
Marsupialia have a true allantoic placenta. The pla- 



XI.] DISCOIDAL PLACENTA. 353 

centa presents a great variety of forms, and we propose 
first to treat the most important of these in succession, 
and then to give a general exposition of their mutual 
affinities. 

The discoidal placenta is found in the Rodentia, 
Insectivora, and Cheiroptera. The Rabbit may be 
taken as an example of this type of placenta. 

The Rabbit. In the pregnant female Rabbit several ova are 
generally found in each horn of the uterus. The general condi- 
tion of the foetal-membranes at the time of their full development 
is shewn in Fig. 116. 

The embryo is surrounded by the amnion, which is compara- 
tively small. The yolk-sac (ds) is large and attached to the 
embryo by a long stalk. It has the form of a flattened sac 
closely applied to about two-thirds of the surface of the subzonal 
membrane. The outer wall of this sac, adjoining the subzonal 
membrane, is formed of hypoblast only ; but the inner wall is 
covered by the mesoblast of the area vaaculosa, as indicated by 
the thick black line (fd). The vascular area is bordered by 
the sinus terminalis (st). In an earlier stage of development the 
yolk-sac had not the compressed form represented in the figure. 
It is, however, remarkable that the vascular area never extends 
over the whole yolk-sac ; but the inner vascular wall of the yolk- 
sac fuses with the outer wall, and with the subzonal membrane, 
and so forms a false chorion, which receives its blood supply 
from the yolk-sac. This part of the chorion does not develop 
vascular villi. 

The allantois (al) is a simple vascular sac with a large cavity. 
Part of its wall is applied to the subzonal membrane, and gives rise 
to the true chorion from which there project numerous vascular 
villi. These fit into corresponding uterine crypts. It seems pro- 
bable, from BischofFs and Kolliker's observations, that the sub- 
zonal membrane in the area of the placenta becomes attached, 
by means of villi, to the uterine wall even before its fusion with 
the allantois. In the later periods of gestation the intermingling 
of the maternal and fcetal parts of the placenta becomes very 

F. & B. 23 



354 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. 

close, and the placenta is truly deciduate. The cavity of the 
allantois persists till birth. Between the yolk-sac, the allantois, 
and the embryo, there is left a large cavity filled with an albumi- 
nous fluid. 

FIG. 116. 




DIAGRAMMATIC LONGITUDINAL SECTION OF A RABBIT'S OVUM 
AT AN ADVANCED STAGE OF PREGNANCY. (From Kolliker 
after Bischoff.) 

e. embryo ; a. amnion ; a. urachus ; al. allantois with blood- 
vessels ; sh. sub-zonal membrane ; pi. placental villi ; fd. 
vascular layer of yolk-sac; ed. hypoblastic layer of yolk- 
sac ; ed'. inner portion of hypoblast, and ed". outer portion 
of hypoblast lining the compressed cavity of the yolk-sac ; 
ds. cavity of yolk-sac ; st. sinus terminalis ; r. space filled 
with fluid between the amnion, the allantois and the yolk- 
sac. 

The metadiscoidal type of placenta is found in 
Man and the Apes. The placenta of Man may be con- 
veniently taken as an example of this type. 



XI.] METADISCOIDAL PLACENTA. 355 

Man. The early stages in the development of the foetal 
membranes in the human embryo have not been satisfactorily 
observed ; but it is known that the ovum, shortly after its 
entrance into the uterus, becomes attached to the uterine wall, 
which in the meantime has undergone considerable preparatory 
changes. A fold of the uterine wall appears to grow round the 
blastodermic vesicle, and to form a complete capsule for it, but 
the exact mode of formation of this capsule is a matter of infer- 
ence and not of observation. During the first fortnight of preg- 
nancy villi grow out, over the whole surface of the ovum. The 
further history of the early stages is extremely obscure : what 
is known with reference to it will be found on p. 335 et seq. ; we 
will here take up the history at about the fourth week. 

At this stage a complete chorion has become formed, and is 
probably derived from a growth of the niesoblast of the allantois 
(unaccompanied by the hypoblast) round the whole inner surface 
of the subzonal membrane. From the whole surface of the 
chorion there project branched vascular processes, covered by 
an epithelium. The allantois is without a cavity, but a hypo- 
blastic epithelium is present in the allantoic stalk, though 
not forming a continuous tube. The blood-vessels of the 
chorion are derived from the usual allantoic arteries and vein. 
The general condition of the embryo and of its membranes at 
this period is shewn diagrammatically in Fig. 114, 5. Around 
the embryo is seen the amnion, already separated by a consider- 
able interval from the embryo. The yolk-sac is shewn at ds. 
Eelatively to the other parts it is considerably smaller than 
it was at an earlier stage. The allantoic stalk is shewn at al. 
Both it and the stalk of the yolk-sac are enveloped by the 
amnion, am. The chorion with its vascular processes surrounds 
the whole embryo. 

It may be noted that the condition of the chorion at this 
stage is very similar to that of the normal diffused type of pla- 
centa, described in the sequel. 

While the above changes are taking place in the embryonic 
membranes, the blastodermic vesicle greatly increases in size, and 
forms a considerable projection from the upper wall of the 
uterus. Three regions of the uterine wall, in relation to the 

232 



356 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. 

blastodermic vesicle, are usually distinguished; and since the 
superficial parts of all of these are thrown off with the after- birth, 
each of them is called a decidua. They are represented at a 
somewhat later stage in Fig. 117. There is (1) the part of the 
wall reflected over the blastodermic vesicle, called the decidua 
reflexa (dr) ; (2) the part of the wall forming the area round 
which the reflexa is inserted, called the decidua serotina (ds) ; (3) 
the general wall of the uterus, not related to the embryo, called 
the decidua vera (du). 

The decidua reflexa and serotina together envelop the chorion 
(Fig. 114. 5), the processes of which fit. into crypts in them. 
At this period both of them are highly and nearly uniformly 
vascular. The general cavity of the uterus is to a large extent 
obliterated by the ovum, but still persists as a space filled with 
mucus, between the decidua reflexa and the decidua vera. 

The changes which ensue from this period onwards are fully 
known. The amnion continues to dilate (its cavity being tensely 
filled with amniotic fluid) till it comes very close to the chorion 
(Fig. 117, am); from which, however, it remains separated by a 
layer of gelatinous tissue. The villi of the chorion in the region 
covered by the decidua reflexa, gradually cease to be vascular, 
and partially atrophy, but in the region in contact with the 
decidua serotina increase and become more vascular and more 
arborescent (Fig. 117, z). The former region becomes known as 
the chorion Iceve, and the latter as the chorion frondosum. The 
chorion frondosum, together with the decidua serotina, gives rise 
to the placenta. 

The umbilical vesicle (Fig. 117, rib\ although it becomes 
greatly reduced in size and flattened, persists in a recognisable 
form till the time of birth. 

The decidua reflexa, by the disappearance of the vessels in the 
chorion Iseve, becomes non-vascular. Its tissue and that of the 
decidua vera undergo changes which we do not propose to 
describe here ; it ultimately fuses on the one hand with the 
chorion, and on the other with the decidua vera. The mem- 
brane resulting from its fusion with the latter structure becomes 
thinner and thinner as pregnancy advances, and is reduced to a 
thin layer at the time of birth. 



XL] 



THE CHORION. 

FIG. 117. 



du. 




DIAGRAMMATIC SECTION OF PREGNANT HUMAN UTERUS WITH 
CONTAINED F<ETU8. (From Huxley after Longet.) 

<il. allantoic stalk ; nb. umbilical vesicle ; am. amnion ; ch. cho- 
rion ; <&. decidua serotina ; du. decidua vera ; dr. decidua 
reflexa ; I. fallopian tube ; c. cervix uteri ; u. uterus ; z. foetal 
villi of true placenta; ^. villi of non-placental part of 
chorion. 

The placenta has a somewhat discoidal form, with a slightly 
convex uterine surface and a concave embryonic surface. At its 
edge it is continuous both with the decidua reflexa and decidua 
vera. Near the centre of the embryonic surface is implanted the 
umbilical cord. As has already been mentioned, the placenta is 
formed of the decidua serotina and the foetal villi of the chorion 
frondosum. The fcetal and maternal tissues are far more closely 
united than in the placenta of the rabbit. The villi of the 
chorion, which were originally comparatively simple, become 
more and more complicated, and assume an extremely arborescent 
form. At birth the whole placenta, together with the fused de- 



358 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. 

cidua vera, and reflexa, with which it is continuous, is shed ; and 
the blood-vessels thus ruptured are closed by the contraction of 
the uterine walls. 

The metadiscoidal placenta of Man and Apes and the discoidal 
placenta of the Eabbit are usually classified by anatomists as 
discoidal placentae, but it must be borne in mind that they differ 
very widely. 

In the Eabbit there is a dorsal placenta, which is co- extensive 
with the area of contact between the allantois and the subzonal 
membrane, while the yolk-sac adheres to a large part of the 
subzonal membrane. In Apes and Man the allantois spreads 
over the whole inner surface of the subzonal membrane ; the 
placenta is on the ventral side of the embryo, and occupies only a 
small part of the surface of the allantois. 

Zonary placenta. Another form of deciduate pla- 
centa is known as the zonary. This form of placenta 
occupies a broad zone of the chorion, leaving the two 
poles free. It is found in the Carnivora, Hyrax, Elephas, 
and Orycteropus. 

In the Dog, which may be taken as a type, there is a large 
vascular yolk-sac formed in the usual way, which does not how- 
ever fuse with the chorion. It has at first an oval shape, and 
persists till birth. The allantois first grows out on the dorsal 
side of the embryo, where it coalesces with the subzonal mem- 
brane, over a small discoidal area, and there is thus formed a 
rudimentary discoidal placenta closely resembling that of the 
Rabbit. 

The area of adhesion between the outer part of the allantois 
and subzonal membrane gradually spreads over the whole inte- 
rior of the subzonal membrane, and vascular villi are formed over 
the whole area of adhesion except at the two extreme poles of the 
ovum. 

With the full growth of the allantois there is formed a broad 
placental zone, with numerous branched villi fitting into corre- 
sponding pita which are not true glands but special develop- 



XT.] NON-DECIDUATE PLACENTA. 359 

ments of the uterine surface. The maternal and foetal structures 
become closely interlocked and highly vascular ; and at birth a 
large part of the maternal part is carried away with the placenta ; 
some of it however still remains attached to the muscular wall of 
the uterus. The zone of the placenta diminishes greatly in pro- 
portion to the chorion as the latter elongates, and at the full 
time the breadth of the zone is not more than about one-fifth of 
the whole length of the chorion. 

At the edge of the placental zone there is a very small portion 
of the uterine mucous membrane reflected over the non-placental 
part of the chorion, so as to form a small reflexa analogous with 
the reflexa in Man. 

The most important of the remaining types of pla- 
centa are the diffuse and the polycotyledonary, and 
these placente are for the most part non-deciduate. In 
the diffuse placenta, found in the Horse, Pig, Le- 
murs, etc., the allantois completely envelopes the em- 
bryo, and villi are formed on all parts of the chorion, 
excepting over a small area at the two poles. 

In the polycotyledonary placenta, which is charac- 
teristic of the Ruminantia, the allantois grows round the 
whole inner surface of the subzonal membrane ; the 
placental villi are however not uniformly distributed, 
but collected into patches or cotyledons, which form as 
it were so many small placentae. The foetal villi of 
these patches fit into corresponding pits in thickened 
patches of the wall of the uterus. 

Comparative histology of the Placenta. 

It does not fall within the province of this work to 
treat from a histological standpoint the changes which 
take place in the uterine walls during pregnancy. It 
will, however, be convenient to place before the reader 



360 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. 

a short statement of the relations between the maternal 
and foetal tissues in the different varieties of placenta. 

The simplest known condition of the placenta is 
that found in the pig (Fig. 118 II.). The papilla-like 
foetal villi fit into the maternal crypts. The villi (v) are 
formed of a connective tissue core with capillaries, and 
are covered by a layer of very flat epithelium (e) de- 
rived from the subzonal membrane. The maternal 
crypts are lined by the uterine epithelium (e), imme- 
diately below which is a capillary plexus. The maternal 
and fcetal vessels are here separated by a double epi- 
thelial layer. The same general arrangement holds 
good in the diffused placentae of other forms, and in the 
polycotyledonary placenta of the Ruminantia, but the 
foetal villi in the latter (III.) acquire an arborescent form. 
The maternal vessels retain the form of capillaries. 

In the deciduate placenta a much more compli- 
cated arrangement is usually found. In the typical 
zonary placenta of the fox and cat (IV. and V.), the 
maternal tissue is broken up into a complete trabecuiar 
meshwork, and in the interior of the trabeculse there 
run dilated maternal capillaries (d'). The trabeculse 
are covered by a more or less columnar uterine epi- 
thelium (e), and are in contact on every side with foetal 
villi. The capillaries of the foetal villi preserve their 
normal size, and the villi are covered by a flat epithelial 
layer (e). 

In the Sloth (VI.) which has a discoidal placenta the 
maternal capillaries become still more dilated, and the 
epithelium covering them is formed of very flat poly- 
gonal cells. 



XL] 



HISTOLOGY OF THE PLACENTA. 



361 



FIG. 118. 



II. 




M 




III. 



IV. 



D 




362 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. 

VI. 







X.] HISTOLOGY OF THE PLACENTA. 363 

DIAGRAMMATIC REPRESENTATIONS OF THE MINUTE STRUCTURE 
OF THE PLACENTA. (From Turner.) 

F. the foetal ; M. the maternal placenta ; e. epithelium of cho- 
rion ; e'. epithelium of maternal placenta ; d. foetal blood- 
vessels ; d'. maternal blood-vessels ; v. villus. 

I. Placenta in its most generalized form. II. Structure of 
placenta of a Pig. III. Of a Cow. IV. Of a Fox. V. Of a 
Cat. 

VI. Structure of placenta of a Sloth. On the right side of 
the figure the flat maternal epithelial cells are shewn in situ. 
On the left side they are removed, and the dilated maternal vessel 
with its blood-corpuscles is exposed. 

VII. Structure of Human placenta. In addition to the let- 
ters already referred to, ds, ds. represents the decidua serotina of 
the placenta ; t, t. trabeculae of serotina passing to the foetal villi ; 
ca. curling artery ; up. utero-placental vein ; x. a prolongation of 
maternal tissue on the exterior of the villus outside the cellular 
layer e', which may represent either the endothelium of the 
maternal blood-vessel or delicate connective tissue belonging to 
the serotina, or both. The layer e' represents maternal cells 
derived from the serotina. The layer of foetal epithelium cannot 
be seen on the villi of the fully-formed human placenta. 

In the human placenta (VII.), as in that of Apes, 
the greatest modification is found. Here the maternal 
vessels have completely lost their capillary form, and 
have become expanded into large freely communicating 
sinuses (d f ). In these sinuses the foetal villi hang for 
the most part freely, though occasionally attached to 
their walls by strands of tissue (t). In the late stages 
of fcetal life there is only one epithelial layer (e} be- 
tween the maternal and fcetal vessels, which closely 
invests the fcetal villi, but is part of the uterine tissue. 
In the foetal villi the vessels retain their capillary form. 



364< EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. XI. 

Evolution of the placenta. Excluding the mar- 
supials whose placentation is not really known, the 
arrangement of the foetal membranes of the Rabbit is 
the most primitive observed. In this type the allantois 
and yolk-sac both function in obtaining nutriment 
from the mother ; and the former occupies only a small 
discoidal area of the subzonal membrane. In all higher 
types the allantois gradually spreads out over the whole 
inner surface of the subzonal membrane and its im- 
portance increases ; while that of the yolk-sac as a nu- 
tritive organ decreases. In the diffuse type of placenta 
simple villi are present over nearly the whole surface of 
the chorion. In the remaining types the villi become 
more complicated and restricted to a smaller area 
(meta-discoidal, zonary, &c.) of the chorion ; though in 
the early stages they are more scattered and simpler, 
in some cases occupying nearly the whole surface of the 
chorion. It therefore seems probable that the placenta 
of Man has been derived not directly from the discoidal 
placenta of the Rabbit, but from the diffuse placenta 
such as is seen in the Lemurs, etc., and that generally 
the zonary, cotyledonary, &c. types of placenta have 
been derived from the diffuse by a concentration and 
increase in the complexity of the fcetal villi. 



CHAPTER XII. 

THE DEVELOPMENT OF THE ORGANS IN MAMMALIA. 

IN chap, X. we have described the early stages and 
general development of the mammalian embryo. In 
the present chapter we propose to examine the for- 
mation of such mammalian organs as differ in their 
development from those of the chick. This will not be 
a work of any considerable extent, as in all essential 
points the development of the organs in the two groups 
is the same. They will be classified according to the 
germinal layers from which they originate. 

THE ORGANS DERIVED FROM THE EPIBLAST. 

Hairs are formed in solid processes of the deep 
(Malpighian) layer of the epidermis, which project into 
the subjacent dermis. The hair itself arises from a 
cornification of the cells of the axis of one of the above 
processes ; and is invested by a sheath similarly formed 
from the more superficial epidermic cells. A small 
papilla of the dermis grows into the inner end of the 
epidermic process when the hair is first formed. The 



366 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 

first trace of the hair appears close to this papilla, but 
soon increases in length, and when the end of the hair 
projects from the surface, the original solid process of 
the epidermis becomes converted into an open pit, the 
lumen of which is filled by the root of the hair. 

The development of nails has been already described 
on p. 283. 

Glands. The secretory part of the various glandular 
structures belonging to the skin is invariably formed 
from the epidermis. In Mammalia it appears that 
these glands are always formed as solid ingrowths of the 
Malpighian layer. The ends of these ingrowths dilate 
to form the true glandular part of the organs, while the 
stalks connecting the glandular portions with the sur- 
face form the ducts. In the case of the sweat-glands 
the lumen of the duct becomes first established ; its 
formation is inaugurated by the appearance of the 
cuticle, and appears first at the inner end of the duct 
and thence extends outwards. In the sebaceous glands 
the first secretion is formed by a fatty modification of 
the whole of the central cells of the gland. 

The muscular layer of the secreting part of the 
sweat-glands is said to be formed from a modification of 
the deeper layer of the epidermic cells. 

The mammary glands arise in essentially the same 
manner as the other glands of the skin. The glands of 
each side are formed as a solid bud of the Malpighian 
layer of the epidermis. From this bud processes sprout 
out, each of which gives rise to one of the numerous 
glands of which the whole organ is formed. 



XII.] THE HIND BRAIN. 367 

The central nervous system. 

The development of the spinal cord in Mammals 
differs in no important respects from that of the chick, 
and we have nothing to add to the account we have 
already given of its general development and histoge- 
nesis in that animal. The development of the brain 
however will be described at greater length, and some 
additional facts relative to the development of the 
Avian brain will be mentioned. 

The first differentiation of the brain takes place in 
Mammalia before the closure of the medullary folds, 
and results as in the chick in the formation of the three 
cerebral vesicles, the fore-, mid- and hind-brain (Fig. 
106, B). A cranial flexure precisely resembling that of 
the chick soon makes its appearance. 

The hind brain early becomes divided into two 
regions, the rudimentary medulla oblongata and cere- 
bellum. 

The posterior section, the medulla, undergoes changes 
of a somewhat complicated character. In the first place 
its roof becomes very much extended and thinned 
out. At the raphe, where the two lateral halves 
of the brain originally united, a separation, as it were, 
takes place, and the two sides of the brain become 
pushed apart, remaining united by only a very thin 
layer of nervous matter, consisting of a single row of 
flattened cells (Fig. 40). As a result of this peculiar 
growth in the brain, the roots of the nerves of the two 
sides, which were originally in contact at the dorsal 
summit of the brain, become carried away from one 
another, and appear to rise at the sides of the brain. 



368 DEVELOPMENT OF OEGANS IN MAMMALIA. [CHAP, 

The thin roof of the fourth ventricle thus formed 
is somewhat rhomboidal in shape. 

At a later period the blood-vessels of the pia 
mater form a rich plexus over the anterior part of 
this thin roof which becomes at the same time some- 
what folded. The whole structure is known as the 
tela vasculosa or choroid plexus of the fourth ventricle 
(Fig. 119, chd 4). The floor of the whole hind -brain 
becomes thickened, and there very soon appears on its 
outer surface a layer of longitudinal non-medullated 
nerve-fibres, similar to those which first appear on the 
spinal cord (p. 252). They are continuous with a similar 
layer of fibres on the floor of the mid-brain, where 
they constitute the crura cerebri. On the ventral floor 
of the fourth ventricle is a shallow continuation of the 
anterior fissure of the spinal cord. 

Subsequently to the longitudinal fibres already spoken of, 
there develope first the olivary bodies of the ventral side of the 
medulla, and at a still later period the pyramids. The fasciculi 
teretes in the cavity of the fourth ventricle are developed shortly 
before the pyramids. 

When the hind-brain becomes divided into two 
regions the roof of the anterior part does not become 
thinned out like that of the posterior, but on the con- 
trary, becomes somewhat thickened and forms a band- 
like structure roofing over the anterior part of thc k 
fourth ventricle (Fig. 39 c6). 

This is a rudiment of the cerebellum, and in all 
Craniate Vertebrates it at first presents this simple 
structure and insignificant size. 

In Birds the cerebellum attains a very considerable 
development (Fig. 119 cbl), consisting of a folded central 



XII.] THE HIND-BKAIN. 369 

lobe with an arbor vitse, into which the fourth ventricle 
is prolonged. There are two small lateral lobes, ap- 
parently equivalent to the flocculi. 

In Mammalia the cerebellum attains a still greater 
development. The median lobe or vermiform process 

FIG. 119. 




LONGITUDINAL SECTION THROUGH THE BRAIN OF A CHICK OF 
TEN DAYS. (After Mihalkovics.) 

hms. cerebral hemispheres ; alf. olfactory lobe ; alf^ olfactory 
nerve ; ggt. corpus striatum ; oma. anterior commissure ; 
did 3. choroid plexus of the third ventricle ; pin. pineal 
gland ; cmp. posterior commissure ; trm. lamina terminalis ; 
chm. optic chiasma ; inf. infundibulum ; hph. pituitary body ; 
bgm. commissure of Sylvius (roof of iter a tertio ad quartum 
ventriculum) ; vma. velum medullse anterius (valve of Vieus- 
sens) ; cbl. cerebellum ; chd 4. choroid plexus of the fourth 
ventricle ; obt 4. roof of fourth ventricle ; obi. medulla oblon- 
gata ; pns. commissural part of medulla ; inv. sheath of 
brain ; bis. basilar artery ; crts. internal carotid. 

F. & B. 24 



370 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 

is first developed. In the higher Mammalia the lateral 
parts constituting the hemispheres of the cerebellum 
become formed as swellings at the sides at a consider- 
ably later period; these are hardly developed in the 
Monotremata and Marsupialia. 

The cerebellum is connected with the roof of the mid-brain in 
front and with the choroid plexus of the fourth ventricle behind 
by delicate membranous structures, known as the velum me- 
dullse anterius (valve of Yieussens) (Fig. 119 vma) and the velum 
medullse posterius. 

The pons Varolii is formed on the ventral side of the floor of 
the cerebellar region as a bundle of transverse fibres at about the 
same time as the olivary bodies. It is represented in Birds by 
a small number of transverse fibres on the floor of the hind-brain 
immediately below the cerebellum. 

The mid-brain. The changes undergone by the 
mid-brain are simpler than those of any other part of 
the brain. It forms, on the appearance of the cranial 
flexure, an unpaired vesicle with a vaulted roof and^ 
curved floor, at the front end of the long axis of the 
body (Fig. 67, MB). It is at this period in Mammalia 
as well as in Aves relatively much larger than in the 
adult: its cavity is known as the iter a tertio ad 
quartum ventriculum or aqueductus Sylvii. 

The roof of the mid-brain is sharply constricted 
off from the divisions of the brain in front of and 
behind it, but these constrictions do not extend to the 
floor. 

In Mammalia the roof and sides give rise to two 
pairs of prominences, the corpora quadrigemina. 

These prominences, which are simply thickenings 
not containing' any prolongations of the iter, become 



XII.] THE FOKE-BKAIN. 371 

first visible on the appearance of an oblique transverse 
furrow, by which the whole mid-brain is divided into an 
anterior and posterior portion. The anterior portion is 
further divided by a longitudinal furrow into the two 
anterior tubercles (nates) ; but it is not until later on 
that the posterior portion is similarly divided longitu- 
dinally into the two posterior tubercles (testes). 

The floor of the mid -brain, bounded posteriorly by 
the pons Varolii, becomes developed and thickened into 
the crura cerebri. The corpora geniculata interna also 
belong to this division of the brain. 

Fore-brain. The early development of the fore- 
brain in Mammals is the same as in the chick. It forms 
at first a single vesicle without a trace of separate 
divisions, but very early buds off the optic vesicles, 
whose history is described with that of the eye. The 
anterior part becomes prolonged and at the same time 
somewhat dilated. At first there is no sharp boundary 
between the primitive fore-brain and its anterior 
prolongation, but there shortly appears a constriction 
which passes from above obliquely forwards and down- 
wards. 

Of these two divisions the posterior becomes the 
thalamencephalon, while the anterior and larger division 
forms the rudiment of the cerebral hemispheres (Fig. 
39 cer) and olfactory lobes. For a considerable period 
this rudiment remains perfectly simple, and exhibits no 
signs, either externally or internally, of a longitudinal 
constriction dividing it into two lobes. 

The thalamencephalon forms at first a simple 
vesicle, the walls of which are of a nearly uniform thick- 
ness and formed of the usual spindle-shaped cells. 

242 



372 DEVELOPMENT OF OEGANS IN MAMMALIA. [CHAP. 

The cavity it contains is known as the third ventricle. 
Anteriorly it opens widely into the cerebral rudiment, 
and posteriorly into the ventricle of the mid-brain. 
The* opening into the cerebral rudiment becomes the 
foramen of Monro. 

For convenience of description we may divide the 
thalamencephalon into three regions, viz. (1) the floor, 
(2) the sides, and (3) the roof. 

The floor becomes divided into two parts: an an- 
terior part, giving origin to the optic nerves, in which is 
formed the optic chiasma ; and a posterior part, which 
becomes produced into a prominence at first incon- 
spicuous the rudiment of the infundibulum (Fig. 39 In). 
This cornes in contact with the involution from the 
mouth which gives rise to the pituitary body (Fig. 
39 pt). 

In Birds, although there is a close connection be- 
tween the pituitary body and the infundibulum, there 
is no actual fusion of the two. In Mammalia the case 
is different. The part of the infundibulum which lies 
at the hinder end of the pituitary body is at first a 
simple finger-like process of the brain (Fig. 120 inf)\ 
but its end becomes swollen, and the lumen in this 
part becomes obliterated. Its cells, originally similar to 
those of the other parts of the nervous system, and even 
containing differentiated nerve-fibres, partly atrophy 
and partly assume an indifferent form, while at the 
same time there grow in amongst them numerous 
vascular and connective-tissue elements. The process 
of the infundibulum thus metamorphosed becomes in- 
separably connected with the true pituitary body, of 
which it is usually described as the posterior lobe. 



XII.] THE THALAMENCEPHALON. 373 

In the later stages of development the unchanged 
portion of the infundibulum becomes gradually pro- 
longed and forms an elongated diverticulum of the 
third ventricle, the apex of which is in contact with 
the pituitary body (Fig. 120 hph). 

The posterior part of the primitive infundibulum becomes the 
corpus albicans, which is double in Man and the higher Apes ; 
the ventral part of the posterior wall forms the tuber cinereum. 
Laterally, at the junction of the optic thalami and infundibulum, 
there are continued some of the fibres of the crura cerebri, which 
arc probably derived from the walls of the infundibulum. 

The sides of the thalamencephalon become very 
early thickened to form the optic thalami, which con- 
stitute the most important section of the thalamen- 
cephalon. These are separated on their inner aspect 
from the infundibular region by a somewhat S-shaped 
groove, known as the sulcus of Monro, which ends in 
the foramen of Monro. They also become secondarily 
united by a transverse commissure, the grey or middle 
commissure, which passes across the cavity of the third 
ventricle. 

The roof undergoes more complicated changes. It 
becomes divided, on the appearance of the pineal gland 
as a sm^all papilliform outgrowth (the development of 
which is dealt with below), into two regions a longer 
anterior in front of the pineal gland, and a shorter pos- 
terior. The anterior region becomes at an early period 
excessively thin, and at a later period, when the roof of 
the thalamencephalon is shortened by the approach of 
the cerebral hemispheres to the mid-brain, it becomes 
(vide Fig. 120 did 3) considerably folded, while at the 
same time a vascular plexus is formed in the pia mater 



374 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP, 
FIG. 120. 




LONGITUDINAL VERTICAL SECTION THROUGH THE ANTERIOR 
PART OP THE BRAIN OF AN EMBRYO RABBIT OF FOUR 
CENTIMETRES. (After Mihalkovics.) 

The section passes through the median line so that the cere- 
bral hemispheres are not cut ; their position is however indicated 
in outline. 

spt. septum lucidum formed by the coalescence of the inner walls 
of part of the cerebral hemispheres ; cma. anterior com- 
missure ; frx. vertical pillars of the fornix ; cat. genu of 
corpus callosum ; trm. lamina terminalis ; hms. cerebral 
hemispheres ; olf. olfactory lobes ; ad. artery of corpus 
callosum ; fmr. position of foramen of Monro ; chd 3. choroid 
plexus of third ventricle ; pin. pineal gland ; cmp. posterior 
commissure ; bgm. lamina uniting the lobes of the mid- 
brain ; chm. optic chiasma ; hph. pituitary body ; inf. infun- 
dibulum ; pns. pons Varolii ; pde. cerebral peduncles ; agd. 
iter a tertio ad quartum ventriculum. 



XII.] THE PINEAL GLAND. 375 

above it. On the accomplishment of these changes it 
is known as the tela choroidea of the third ventricle. 

In the roof of the third ventricle behind the pineal 
gland there appear transverse commissural fibres, form- 
ing a structure known as the posterior commissure, 
which connects together the two optic thalarni. 

The most remarkable organ in the roof of the thala- 
mencephalon is the pineal gland, which is developed as 
a hollow papilliform outgrowth of the roof, and is at 
first composed of cells similar to those of the other 
parts of the central nervous system (Fig. 120 pin). It 
is directed backwards over the hinder portion of the 
roof of the thalamencephalon. 

In Birds (p. 116) the primitive outgrowth to form 
the pineal gland becomes deeply indented by vascular 
connective-tissue ingrowths, so that it assumes a den- 
dritic structure (Fig. 119 pin). The proximal extremity 
attached to the roof of the thalamencephalon soon 
becomes solid and forms a special section, known as 
the infra-pineal process. The central lumen of the 
free part of the gland finally atrophies, but the branches 
still remain hollow. The infra-pineal process becomes 
reduced to a narrow stalk, connecting the branched 
portion of the body with the brain. 

In Mammalia the development of the pineal gland 
is generally similar to that of Birds. The original out- 
growth becomes branched, but the follicles or lobes to 
which the branching gives rise eventually become solid 
(Fig. 120 pin). An infra-pineal process is developed 
comparatively late, and is not sharply separated from 
the roof of the brain. 

No satisfactory suggestions have yet been offered as 



376 DEVELOPMENT OF OKGANS IN MAMMALIA. [CHAP. 

to the nature of the pineal gland. It appears to possess 
in all forms an epithelial structure, but, except at the 
base of the stalk (infra-pineal process) in Mammalia, in 
the wall of which there are nerve-fibres, no nervous 
structures are present in it in the adult state. 

The cerebral hemispheres. It will be convenient 
to treat separately the development of the cerebral 
hemispheres proper, and that of the olfactory lobes. 

In the cerebral rudiment two parts may be dis- 
tinguished, viz. the floor and the roof. The former gives 
rise to the ganglia at the base of the hemispheres, the 
corpora striata, the latter to the hemispheres proper. 

The first change which takes place consists in the 
roof growing out into two lobes, between which a shallow 
median constriction makes "its appearance (Fig. 121). 




ce 



DIAGRAMMATIC LONGITUDINAL HORIZONTAL SECTION THROUGH 
THE FORE-BRAIN. 

3.v. third ventricle ; Iv. lateral ventricle ; It. lamina terminalis ; 
ce. cerebral hemisphere ; op. th. optic thalamus. 



XII.] THE CEREBRAL HEMISPHERES. 377 

The two lobes thus formed are the rudiments of the 
two hemispheres. The cavity of each of them opens 
by a widish aperture into a cavity at the base of the 
cerebral rudiment, which again opens directly into the 
cavity of the third ventricle (3 v). The Y-shaped aper- 
ture thus formed, which leads from the cerebral hemi- 
spheres into the third ventricle, is the foramen of 
Monro. The cavity (Iv) in each of the rudimentary 
hemispheres is a lateral ventricle. The part of the 
cerebrum which lies between the two hemispheres, and 
passes forwards from the roof of the third ventricle 
round the end of the brain to the optic chiasma below, 
is the rudiment of the lamina terminalis (Figs. 121 It 
and 123 trm). Up to this point the development of 
the cerebrum is similar in all Vertebrata, and in some 
forms it practically does not proceed much further. 

The cerebral hemispheres undergo in Mammalia the 
most complicated development. The primitive un- 
paired cerebral rudiment becomes, as in lower Ver- 
tebrates, bilobed, and at the same time divided by the 
ingrowth of a septum of connective tissue into two 
distinct hemispheres (Figs. 125 and 124 / and 122 i). 
From this septum is formed the falx cerebri and other 
parts. 

The hemispheres contain at first very large cavities, 
communicating by a wide foramen of Monro with the 
third ventricle (Fig. 124). They grow rapidly in size, 
and extend, especially backwards, and gradually cover 
the thalamencephalon and the mid-brain (Fig. 122 i,f). 
The foramen of Monro becomes very much narrowed 
and reduced to a mere slit. 

The walls are at first nearly uniformly thick, but 



378 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 



FIG. 122. 
/ 2. 





BRAIN OF A THREE MONTHS' HUMAN EMBRYO : NATURAL SIZE. 
(From Kolliker.) 

1. From above with the dorsal part of hemispheres and mid- 
brain removed ; 2. From below. /. anterior part of cut wall 
of the hemisphere ; /'. cornu ammonis ; tho. optic thalamus ; 
cst. corpus striatum ; to. optic tract ; cm. corpora mammil- 
laria ; p. pons Yarolii. 

the floor becomes thickened on each side, and gives rise 
to the corpus striatum (Figs. 124 and 125 st}. The 
corpus striatum projects upwards into each lateral ven- 
tricle, and gives to this a somewhat semilunar form, the 
two horns of which constitute the permanent anterior 
and descending cornua of the lateral ventricles (Fig. 126 
st). 

With the further growth of the hemisphere the cor- 
pus striatum loses its primitive relations to the de- 
scending cornu. The reduction in size of the foramen 
of Monro above mentioned is, to a large extent, caused 
by the growth of the corpora striata. 

The corpora striata are united at their posterior 
border with the optic thalami. In the later stages of 
development the area of contact between these two 
pairs of ganglia increases to a large extent (Fig. 125), 



XII.] 



THE CORPORA STRIATA, 



371) 



and the boundary between them becomes somewhat 
obscure, so that the sharp distinction which exists 
in the embryo between the thalamencephalon and 
cerebral hemispheres becomes lost. 




TRANSVERSE SECTION THROUGH THE BRAIN OF A RABBIT 
FIVE CENTIMETRES. (After Mihalkovics.) 



OF 



The section passes through nearly the posterior border of the 
septum lucidum, immediately in front of the foramen of Monro. 

hms. cerebral hemispheres ; cal. corpus callosum ; amm. cornu 
ammonis (hippocampus major) ; cms. superior commissure 
of the cornua ammonis ; spt. septum lucidum ;/r#2. anterior 
pillars of the fornix ; cma. anterior commissure ; trm. lamina 
terminalis ; sir. corpus striatum ; Itf. nucleus lenticularis 
of corpus striatum ; vtr 1. lateral ventricle ; vtr 3. third 
ventricle ; ipl. slit between cerebral hemispheres. 



380 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 

The outer wall of the hemispheres gradually thick- 
ens, while the inner wall becomes thinner. In the 
latter, two curved folds, projecting towards the interior 
of the lateral ventricle, become formed. These folds 
extend from the foramen of Monro along nearly the 
whole of what afterwards becomes the descending cornu 
of the lateral ventricle. The upper fold becomes the 
hippocampus major (cornu ammonis) (Figs. 123 amm, 
124 and 125 h, and 126 am). 

The wall of the lower fold becomes very thin, and a 
vascular plexus, derived from the connective-tissue 
septum between the hemispheres, and similar to that of 
the roof of the third ventricle, is formed outside it. It 
constitutes a fold projecting into the cavity of the 
lateral ventricle, and together with the vascular con- 
nective tissue in it gives rise to the choroid plexus of 
the lateral ventricle (Figs. 124 and 125 pi). 

It is clear from the above description that a marginal 
fissure leading into the cavity of the lateral ventricle 
does not exist in the sense often implied in works on 
human anatomy, since the epithelium covering the 
choroid plexus, and forming the true wall of the brain, 
is a continuous membrane. The epithelium of the 
choroid plexus of the lateral ventricle is quite inde- 
pendent of that of the choroid plexus of the third 
ventricle, though at the foramen of Monro the roof of 
the third ventricle is of course continuous with the 
inner wall of the lateral ventricle (Fig. 124 s). The 
vascular elements of the two plexuses form however a 
continuous structure. 

The most characteristic parts of the Mammalian 
cerebrum are the commissures connecting the two 



XII.] 



THE CEREBRAL COMMISSURES. 



381 



hemispheres. These commissures are (1) the anterior 
commissure, (2) the fornix, and (3) the corpus callosum, 
the two latter being peculiar to Mammalia. 




TRANSVERSE SECTION THROUGH THE BRAIN OP A SHEEP'S 
EMBRYO OF 27 CM. IN LENGTH. (From Kolliker.) 

The section passes through the level of the foramen of 
Monro. 

st. corpus striatum ; m. foramen of Monro ; t. third ventricle ; 
pi. choroid plexus of lateral ventricle ; /. falx cerebri ; th. 
anterior part of optic thalamus ; ch. optic chiasma ; o. optic 
nerve ; c. fibres of the cerebral peduncles ; h. cornu am- 
monis ; p. pharynx ; sa. pre- sphenoid bone ; a. orbi to- 
sphenoid bone ; s. points to part of the roof of the brain at 
the junction between the roof of the third ventricle and 
the lamina terminalis ; I. lateral ventricle. 



382 DEVELOPMENT OF OEGANS IN MAMMALIA. [CHAP. 

By the fusion of the inner walls of the hemispheres 
in front of the lamina terminalis a solid septum is 
formed, continuous behind with the lamina terminalis, 




TRANSVERSE SECTION THROUGH THE BRAIN OF A SHEEP'S 
EMBRYO OF 2*7 CM. IN LENGTH. (From KOlliker.) 

The section is taken a short distance behind the section 
represented in Fig. 124, and passes through the posterior part of 
the hemispheres and the third ventricle. 

st. corpus striatum ; ih. optic thalamus ; to. optic tract ; t. 

ventricle ; d. roof of third ventricle ; c. fibres of cerebr 
peduncles ; c. divergence of these fibres into the walls of 1 
hemispheres ; e. lateral ventricle with choroid plexus pi ; 
h. cornu ammonis ; /. primitive falx ; am. alisphenoid ; 
orbito-sphenoid ; sa. presphenoid ; p. pharynx ; mk. Meckel's 
cartilage. 



XII.] THE CORPUS CALLOSUM. 383 

and below with the corpora striata (Figs. 120 and 123 spt). 
It is by a series of differentiations within this septum, 
the greater part of which gives rise to the septum luci- 
dum, that the above commissures originate. In Man 
there is a closed cavity left in the septum known as the 
fifth ventricle, which has however no communication 
with the true ventricles of the brain. 

In this septum there become first formed, below and 
behind, the transverse fibres of the anterior commissure 
(Fig. 120 and Fig. 123 cma), while above and behind 
these the vertical fibres of the fornix are developed 
(Fig. 120 and Fig. 123 frx 2). The vertical fibres meet 
above the foramen of Monro, and thence diverge back- 
wards, as the posterior pillars, to lose themselves in the 
cornu ammonis (Fig. 123 amm). Ventrally they are 
continued, as the descending or anterior pillars of the 
fornix, into the corpus albicans, and thence into the 
optic thalami 1 . 

The corpus callosum is not formed till after the 
anterior commissure and fornix. It arises in the upper 
part of the septum formed by the fusion of the lateral 
walls of the hemispheres (Figs. 120 and 123 cal\ and 
at first only its curved anterior portion the genu 01 
rostrum is developed. This portion is alone found 
in Monotremes and Marsupials. The posterior portion, 
which is present in all the Monodelphia, is gradually 
formed as the hemispheres are prolonged further back- 
wards. 

1 Recent observations tend to show that the anterior pillars of the 
fornix end in the corpus albicans ; and that the fibres running from 
the latter into the optic thalami are independent of the anterior 
pillars. 



384 DEVELOPMENT OF OKGANS IN MAMMALIA. [CHAP. 

Primitively the Mammalian cerebrum, like that of 
the lower Vertebrata, is quite smooth. In some of the 
Mammalia, Monotremata, Insectivora, etc., this condition 
is retained nearly throughout life, while in the majority of 
Mammalia a more or less complicated system of fissures 




LATERAL VIEW OF THE BRAIN OF A CALF EMBRYO OF 5 CM. 
(After Mihalkovics.) 

The outer wall of the hemisphere is removed, so as to give a 
view of the interior of the left lateral ventricle. 

hs. cut wall of hemisphere ; st. corpus striatum ; am. hippo- 
campus major (cornu ammonis) ; d. choroid plexus of lateral 
ventricle ; fm. foramen of Monro ; op. optic tract ; in. in- 
fundibulum ; mb. mid-brain ; cb. cerebellum ; IV. V. roof of 
fourth ventricle ; ps. pons Varolii, close to which is the fifth 
nerve with Gasserian ganglion. 

is developed on the surface. The most important, and 
first formed, of these is the Sylvian fissure. It arises at 
the time when the hemispheres, owing to their growth 
in front of and behind the corpora striata have assumed 
somewhat the form of a bean. At the root of the 
hemispheres the hilus of the bean there is formed a 



XII.] HISTOGENESIS. 385 

shallow depression which constitutes the first trace of 
the Sylvian fissure. The part of the brain lying in this 
fissure is known as the island of Reil. 

The fissures of the cerebrum may be divided into two classes ; 
(1) the primitive, (2) the secondary fissures. The primitive fissures 
are the first to appear ; they owe their origin to a folding of the 
entire wall of the cerebral vesicles. Many of them are transient 
structures and early disappear. The most important of those 
which persist are the hippocainpal, the parieto-occipital, the 
calcarine (in Man and Apes) sulci and the Sylvian fissures. 
The secondary fissures appear later, and are due to folds which 
implicate the cortex of the hemispheres only. 

The olfactory lobes. The olfactory lobes, or rhinen- 
cephala, are secondary outgrowths of the cerebral hemi- 
spheres, and contain prolongations of the lateral ven- 
tricles, which may however be closed in the adult state ; 
they arise at a fairly early stage of development from 
the under and anterior part of the hemispheres (Fig. 
127). 

Histogenetic changes. The walls of the brain are 
at first very thin and, like those of the spinal cord, are 
formed of a number of ranges of spindle-shaped cells. 
In the floor of the hind- and mid-brain a superficial 
layer of delicate nerve-fibres is formed at an early 
period. This layer appears at first on the floor and 
sides of the hind-brain, and almost immediately after- 
wards on the floor and the sides of the mid-brain. 
The cells internal to the nerve-fibres become differen- 
tiated into an innermost epithelial layer lining the 
cavities of the ventricles, and an outer layer of grey 
matter. 

The similarity of the primitive arrangement and 
F. & B. 25 



386 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 




SECTION THROUGH THE BRAIN AND OLFACTORY ORGAN OF AN 
EMBRYO OF SCYLLIUM. 

ch. cerebral hemispheres ; ol.v. olfactory vesicle ; off. olfactory 
pit ; Sch. Schneiderian folds ; 1. olfactory nerve (the reference 
line has been accidentally carried through the nerve so as to 
appear to indicate the brain) ; pn. anterior prolongation of 
pineal gland. 

histological characters of the parts of the brain behind 
the cerebral hemispheres to those of the spinal cord is 
very conclusively shewn by the examination of any good 
series of sections. In both brain and spinal cord the 
white matter forms a cap on the ventral and lateral 
parts some considerable time before it extends to the 
dorsal surface. In the medulla oblongata the white 
matter does not eventually extend to the roof owing to 
the peculiar degeneration which that part undergoes. 

In the case of the fore-brain the walls of the hemi- 
spheres become first divided (Kolliker) into a superficial 
thinner layer of rounded elements, and a deeper and 
thicker epithelial layer, and between these the fibres of 



xii.] THE EYE. 387 

the crura cerebri soon interpose themselves. At a 
slightly later period a thin superficial layer of white 
matter, homologous with that of the remainder of the 
brain, becomes established. 

The inner layer, together with the fibres from the 
crura cerebri, gives rise to the major part of the white 
matter of the hemispheres and to the epithelium lining 
the lateral ventricles. 

The outer layer of rounded cells becomes divided 
into (1) a superficial part with comparatively few cells, 
which, together with its coating of white matter, forms 
the outer part of the grey matter, and (2) a deeper 
layer with numerous cells, which forms the main mass 
of the grey matter of the cortex. 

The eyes. The development of the Mammalian eye 
is essentially similar to that of the chick (ch. vi.) There 
are however two features in its development which de- 
serve mention. These are (1) the immense foetal develop- 
ment of the blood-vessels of the vitreous humour and 
the presence in the embryo of a vascular membrane sur- 
rounding the lens, known as the membrana capsulo- 
pujnllaris, (2) the absence of any structure comparable 
to the pecten, and the presence of the arteria centralis 
retinae. 

In the invagination of the lens (rabbit) a thin 
layer of mesoblast is carried before it, and is thus 
transported into the cavity of the vitreous humour. 
In the folding in of the optic vesicle which accom- 
panies the formation of the lens the optic nerve is 
included, and on the development of the cavity of the 
vitreous humour an artery, running in the fold of 
the optic nerve, passes through the choroid slit into the 

252 



388 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 

cavity of the vitreous humour (Fig. 128 acr). The sides 
of the optic nerve subsequently bend over, and com- 
pletely envelope this artery, which then gives off 



[>/*. n - 




SECTION THROUGH THE EYE OF A RABBIT EMBRYO OF ABOUT 
TWELVE DAYS. 

c. epithelium of cornea : 1. lens ; mec. mesoblast growing in from 
the side to form the cornea ; rt. retina ; a.c.r. arteria cen- 
tralis retinse ; of.n. optic nerve. 

The figure shews (1) the absence at this stage of mesoblast 
between the lens and the epiblast ; the interval between the 
two has however been made too great ; (2) the arteria centralis 
retinae forming the vascular capsule of the lens and continuous 
with vascular structures round the edges of the optic cup. 



XII.] MEMBRANA CAPSULO-PUPILLARIS. 389 

branches to the retina, and becomes known as the 
arteria centralis retince. It is homologous with the 
arterial limb of the vascular loop projecting into the 
vitreous humour in Birds. 

Before becoming enveloped in the optic nerve tins 
artery is continued through the vitreous humour (Fig. 
128), and when it comes in close proximity to the lens 
it divides into a number of radiating branches, which 
pass round the edge of the lens, and form a vascular 
sheath which is prolonged so as to cover the anterior 
wall of the lens. In front of the lens they anastomose 
with vessels, coming from the iris, many of which are 
venous, and the whole of the blood from the arteria 
centralis is carried away by these veins. The vascular 
sheath surrounding the lens is the membrana capsulo- 
pupillaris. The posterior part of it is either formed 
simply by branches of the arteria centralis, or out 
of the mesoblast cells involuted with the lens. The 
anterior part of the vascular sheath is however enclosed 
in a very delicate membrane, the membrana pupillaris, 
continuous at the sides with the membrane of Descemet. 

The membrana capsulo-pupillaris is simply a pro- 
visional embryonic structure, subserving the nutrition 
of the lens. 

In many forms, in addition to the vessels of the 
vascular capsule round the lens, there arise from the 
arteria centralis retinae, just after its exit from the optic 
nerve, provisional vascular branches which extend them- 
selves in the posterior part of the vitreous humour. 
Near the ciliary end of the vitreous humour they anas- 
tomose with the vessels of the membrana capsulo-pu- 
pillaris. 



390 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 

The choroid slit closes very early, and is not per- 
forated by any structure homologous with the pecteri. 
The only part of the slit which can be said to remain 
open is that in which the optic nerve is involved ; in the 
Centre of the latter is situated the arteria centralis 
retinae as explained above. From this artery there 
grow out the vessels to supply the retina, which however 
are distinct from the provisional vessels of the vitreous 
humour just described, the blood being returned from 
them by veins accompanying the arteries. On the 
atrophy of the provisional vessels the whole of the blood 
of the arteria centralis passes into the retina. 

Of the cornea, aqueous humour, eyelids and lacrymal 
duct no mention need here be made, the account given in 
Part I. being applicable equally to mammalian embryos. 

The auditory organ. In Mammals, as we have 
seen to be the case in the chick (chap, vi.), the auditory 
vesicle is at first nearly spherical, and is imbedded in 
the mesoblast at the side of the hind-brain. It soon 
becomes triangular in section, with the apex of the tri- 
angle pointing inwards and downwards. This apex 
gradually elongates to form the rudiment of the cochlear 
canal and sacculus hemisphericus (Fig. 129, GO). At 
the same time the recessus labyrinthi (R.L) becomes 
distinctly marked, and the outer wall of the main body 
of the vesicle grows out into two protuberances, which 
form the rudiments of the vertical semicircular canals 
(V.E). In the lower forms (Fig. 132) the cochlear 
process hardly reaches a higher stage of development than 
that found at this stage in Mammalia. 

The parts of the auditory labyrinth thus established 
soon increase in distinctness (Fig. 130); the cochlear 



XII.] THE MEMBRANOUS LABYRINTH. 



391 



Fm. 129. 




TRANSVERSE SECTION OF THE HEAD OF A F(ETAL SHEEP 
(16 MM. IN LENGTH) IN THE REGION OF THE HIND-BRAIN. 
(After Bottcher.) 

HE. the hind-brain. The section is somewhat oblique, hence 
while on the right side the connections of the recessus vestibuli 
R.L.j and of the commencing vertical semicircular canal F.Z?., 
and of the ductus cochlearis CO., with the cavity of the primary 
otic vesicle are seen : on the left side, only the extreme end of the 
ductus cochlearis (7(7, and of the semicircular canal V.B. are shewn. 

Lying close to the inner side of the otic vesicle is seen the 
cochlear ganglion GC ; on the left side the auditory nerve G' and 
its connection N with the hind-brain are also shewn. 

Below the otic vesicle on either side lies the jugular vein. 



392 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 

canal ((7(7) becomes longer and curved ; its inner and 
concave surface being lined by a thick layer of columnar 
epiblast. The recessus labyrinthi also increases in 
length, and just below the point where the bulgings to 
form the vertical semicircular canals are situated, there 
is formed a fresh protuberance for the horizontal semi- 

FIG. 130. 




SECTION OF THE HEAD OP A FOETAL SHEEP 20 MM. IN 
LENGTH. (After Bottcher.) 

R. V. Recessus labyrinthi ; V.B. vertical semicircular canal ; HE. 
horizontal semicircular canal ; C.C. cochlear canal ; G. coch- 
kar ganglion. 



XII.] THE MEMBRANOUS LABYRINTH. 393 

circular canal. At the same time the central parts of 
the walls of the flat bulgings of the vertical canals grow 
together, obliterating this part of the lumen, but leaving 
a canal round the periphery ; and, on the absorption of 
their central parts, each of the original simple bulgings 
of the wall of the vesicle becomes converted into a true 
semicircular canal, opening at its two extremities into 
the auditory vesicle. The vertical canals are first es- 
tablished and then the horizontal canal. 

Shortly after the formation of the rudiment of the 
horizontal semicircular canal a slight protuberance be- 
comes apparent on the inner commencement of the 
cochlear canal. A constriction arises on each side of 
the protuberance, converting it into a prominent hemi- 
spherical projection, the sacculus hemisphericus (Fig. 
131 8E). 

The constrictions are so deep that the sacculus is 
only connected with the cochlear canal on the one hand, 
and with the general cavity of the auditory vesicle on 
the other, by, in each case, a narrow short canal. The 
former of these canals (Fig. 131 6) is known as the 
canalis reuniens. 

At this stage we may call the remaining cavity of 
the original otic vesicle, into which all the above parts 
open, the utriculus. 

Soon after the formation of the sacculus hemispheri- 
cus, the cochlear canal and the semicircular canals 
become invested with cartilage. The recessus labyrinthi 
remains however still enclosed in undifferentiated meso- 
blast. 

Between the cartilage and the parts which it sur- 
rounds there remains a certain amount of indifferent 



394 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 
FIG. 131. 







JOT 



J3TJB 



SECTION THROUGH THE INTERNAL EAR OF AN EMBRYONIC 
SHEEP 28 MM. IN LENGTH. (After Bottcher.) 

D.M. dura mater; R. V. recessus labyrinthi ; H.V.B. posterior 
vertical semicircular canal ; U. utriculus ; H.B. horizontal 



XII.] THE MEMBRANOUS LABYRINTH. 395 

semicircular canal ; b. canalis reunions ; a. constriction by 
means of which the sacculus hemisphericus S.lt. is formed ; 
/. narrowed opening between sacculus hemisphericus and 
utriculus ; C.C. cochlea ; C.C 1 . lumen of cochlea ; K.K. 
cartilaginous capsule of cochlea ; K.B. basilar plate ; C/i. 
notochord. 

connective tissue, which is more abundant around the. 
cochlear canal than around the semicircular canals. 

As soon as they have acquired a distinct connective- 
tissue coat, the semicircular canals begin to bo dilated 
at one of their terminations to form the ampullae. At 
about the same time a constriction appears opposite the 
mouth of the recessus labyrinthi, which causes its open- 
ing to be divided into two branches one towards the 
utriculus and the other towards the sacculus hemispheri- 
cus ; and the relations of the parts become so altered 
that communication between the sacculus and utriculus 
can only take place through the mouth of the recessus 
labyrinthi (Fig. 132). 

When the cochlear canal has come to consist of two 
and a half coils, the thickened epithelium which lines 
the lower surface of the canal forms a double ridge 
from which the organ of Corti is subsequently de- 
veloped. Above the ridge there appears a delicate 
cuticular membrane, the membrane of Corti or mem- 
brana tectoria. 

The epithelial walls of the utricle, the saccule, the 
recessus labyrinthi, the semicircular canals, and the 
cochlear canal constitute together the highly complicated 
product of the original auditory vesicle. The whole 
structure forms a closed cavity, the various parts of 
which are in free communication. In the adult the 



396 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 

fluid present in this cavity is known as the endo- 
lyinph. 

In the mesoblast lying between these parts and the 
cartilage, which at this period envelopes them, lymphatic 
spaces become established, which are partially de- 
veloped in the Sauropsida, but become in Mammals 
very important structures. 

They consist in Mammals partly of a space sur- 
rounding the utricle and saccule and called the vestibule, 
into which open spaces surrounding the semicircular 
canals, and partly of two very definite channels, which 
largely embrace between them the cochlear canal. The 
latter channels form the scala vestibuli on the upper side 
of the cochlear canal and the scala tympani on the lower. 
The scala vestibuli is in free communication with the 
lymphatic cavity surrounding the utricle and saccule, 
and opens at the apex of the cochlea into the scala tyrn- 
pani. The latter ends blindly at the fenestra rotunda. 

The fluid contained in the two scalse, and in the 
remaining lymphatic cavities of the auditory labyrinth, 
is known as perilymph. 

The cavities just spoken of are formed by an absorp- 
tion of parts of the embryonic mucous tissue between 
the perichondrium and the walls of the membranous 
labyrinth. 

The scala vestibuli is formed before the scala tympani, 
and both scalse begin to be developed at the basal end 
of the cochlea : the cavity of each is continually being 
carried forwards towards the apex of the cochlear canal 
by a progressive absorption of the mesoblast. At first 
both scalse are somewhat narrow, but they soon increase 
in size and distinctness. 



XII.] THE COCHLEA. 397 

The cochlear canal, which is often known as the 
scala media of the cochlea, becomes compressed on the 
formation of the scalse so as to be triangular in section, 
with the base of the triangle outwards. This base is 
only separated from the surrounding cartilage by a 
narrow strip of firm mesoblast, which becomes the stria 
vascularis, etc. At the angle opposite the base the coch- 
lear canal is joined to the cartilage by a narrow isthmus 
of firm material, which contains nerves and vessels. This 
isthmus subsequently forms the lamina spiralis, separ- 
ating the scala vestibuli from the scala tympani. 

The scala vestibuli lies on the upper border of the 
cochlear canal, and is separated from it by a very thin 
layer of mesoblast, bordered on the cochlear aspect by 
flat epiblast cells. This membrane is called the mem- 
brane of Reissner. The scala tympani is separated from 
the cochlear canal by a thicker sheet of mesoblast, called 
the basilar membrane, which supports the organ of 
Corti and the epithelium adjoining it. The upper ex- 
tremity of the cochlear canal ends in a blind extremity 
called the cupola, to which the two scalse do not for 
some time extend. This condition is permanent in 
Birds, where the cupola is represented by a structure 
known as the lagena (Fig. 132, II. L). Subsequently 
the two scalse join at the extremity of the cochlear 
canal ; the point of the cupola still however remains in 
contact with the bone, which has now replaced the 
cartilage, but at a still later period the scala vestibuli, 
growing further round, separates the cupola from the 
adjoining osseous tissue. 

Accessory auditory structures. The development 
of the Eustachian tube, tympanic cavity, tympanic 



398 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 
FIG. 132. 




DIAGRAMS OF THE MEMBRANOUS LABYRINTH. (From Gegen- 

baur.) 

I. Fish. II. Bird. III. Mammal 

U. utriculus ; S. sacculus ; US. utriculus and sacculus ; Cr. 
canalis reuniens ; R. recessus labyrinthi ; UC. commence- 
ment of cochlea ; 0. cochlear canal ; L. lagena ; K. cupola 
at apex of cochlear canal ; V. csecal sac of the vestibulum of 
the cochlear canal. 

membrane and external auditory meatus resembles that 
in Birds (p. 166). As in Birds two membranous fenestrse, 
the fenestra ovalis and rotunda, in the bony inner wall of 
the tympanic cavity are formed. The fenestra ovalis 
opens into the vestibule, and is in immediate contiguity 
with the walls of the utricle, while the fenestra rotunda 
adjoins the scala tympani. In place of the columella of 
Birds, three ossicles, the malleus, incus and stapes reach 
across the tympanic cavity from the tympanic membrane 



XII.] 



THE NASAL ORGAN. 



399 



to the fenestra ovalis. These ossicles, which arise 
mainly from the mandibular and hyoid arches (vide 
p. 403), are at first imbedded in the connective tissue in 
the neighbourhood of the tympanic cavity, but on the 
full development of this cavity, become apparently 
placed within it, though really enveloped in the mucous 
membrane lining it. 

Nasal organ. In Mammalia the general formation 
of the anterior and posterior nares is the same as in 
Birds; but an outgrowth from the inner side of the 
canal between the two openings arises at an early period ; 
and becoming separate from the posterior nares and 
provided with a special opening into the mouth, forms 
the organ of Jacobson. The general relations of this 
organ when fully formed are shewn in Fig. 133. 

FIG. 133. 




SECTION THROUGH THE NASAL CAVITY AND JACOBSON'S ORGAN. 

(From Gegenbaur.) 

*n. septum nasi ; en. nasal cavity ; J. Jacobson's organ ; d. edgs 
of upper jaw. 



400 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 

The development of the cranial and spinal 
nerves in Mammals is as far as is known essentially 
the same as in the chick, for an account of which see 
p. 123 et seq. 

Sympathetic nervous system. The development 
of the sympathetic system of both Aves and Mammalia 
has not been thoroughly worked out. There is how- 
ever but little doubt that in Mammalia the main por- 
tion arises in continuity with the posterior spinal 
ganglia. 

The later history of the sympathetic system is inti- 
mately bound up with that of the so-called supra-renal 
bodies, the medullary part of which is, as we shall see 
below, derived from the peripheral part of the sympa- 
thetic system. 

THE ORGANS DERIVED FROM MESOBLAST. 

The vertebral column. The early development of 
the perichordal cartilaginous tube and rudimentary 
neural arches is almost the same in Mammals as in 
Birds. The differentiation into vertebral and inter- 
vertebral regions is the same in both groups; but instead 
of becoming divided as in Birds into two segments 
attached to two adjoining vertebrae, the intervertebral 
regions become in Mammals wholly converted into the 
intervertebral ligaments (Fig. 135 li). There are three 
centres of ossification for each vertebra, two in the arch 
and one in the centrum. 

The fate of the notochord is in important respects 
different from that in Birds. It is first constricted in 
the centres of the vertebrae (Fig. 134) and disappears 
there shortly after the beginning of ossification ; while in 



XII.] 



THE SKULL. 



401 



the intervertebral regions it remains relatively uncon- 
stricted (Figs. 134 and 135 c) and after undergoing 
certain histological changes remains through life as part 
of the nucleus pulposus in the axis of the intervertebral 
ligaments. There is also a slight swelling of the noto- 
chord near the two extremities of each vertebra (Fig. 
135 c and c"}. 

In the persistent vertebral constriction of the notochord 
Mammals retain a more primitive and piscine mode of formation 
of the vertebral column thai} tfye majority either of the Reptilia 
or Amphibia. 

FIG. 134. 




LONGITUDINAL SECTION THROUGH THE VERTEBRAL COLUMN 
OF AN EIGHT WEEKS' HUMAN EMBRYO IN THE THO- 
RACIC REGION. (From Kolliker.) 

v. cartilaginous vertebral body ; li. intervertebral ligament ; 
ch. notochord. 

The skull. Excepting in the absence of the inter- 
orbital plate, the early development of the Mamma- 
lian cranium resembles in all essential points that of 
Aves, to our account of which on p. 235 et seq. we refer 
the reader. 

F. & B. 26 



402 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 
FIG. 135. 



tff C" 



7; 




LONGITUDINAL SECTION THROUGH THE INTERVERTEBRAL LIGA- 
MENT AND ADJACENT PARTS OF TWO VERTEBRA FROM THE 

THORACIC EEGION OF AN ADVANCED EMBRYO OF A SHEEP. 
(From Kolliker.) 

la. .ligamentum longitudinale anterius ; Ip. ligamentum long, pos- 
terius ; li. ligamentum intervertebrale ; , k r . epiphysis of 
vertebra ; w. and w f . anterior and posterior vertebrae ; c. in- 
tervertebral dilatation of notochord ; c.' and c". vertebral di- 
latation of notochord.. 

The early changes in the development of the visceral 
arches and clefts have already been described, but the 
later changes undergone by the skeletal elements of the 
first two visceral arches are sufficiently striking to need 
a special description. 



XII.] MANDIBULAR AND HYOID ARCHES. 403 

The skeletal bars of both the hyoid and mandibular 
arches develop at first more completely than in any 
of the other types above Fishes ; they are articulated to 
each other above, while the pterygo-palatine bar is 
quite distinct. 

The main features of the subsequent development 
are undisputed, with the exception of that of the upper 
end of the hyoid, which is still controverted. The 
following is Parker's account for the Pig. 

The mandibular and hyoid arches are at first very 
similar, their dorsal ends being somewhat incurved, and 
articulating together. 

In a somewhat later stage (Fig. 136) the upper end 
of the mandibular bar (mb), without becoming segmented 

FIG. 13G. 




KMHHYO Pra, AX INCH AND A THIRD LONG ; SIDE VIEW OF 
MANDIBULAR AND HYOID ARCHES. THE MAIN HYOID 
ARCH IS SEEN AS DISPLACED BACKWARDS AFTER SEGMEN- 
TATION FROM THE INCUS. (From Parker.) 

t'j. tongue ; mJc. Meckelian cartilage ; ml. body of malleus ; ml). 
inanubrium or handle of the malleus ; tjy. tegmen tympani ; 
?'. incus ; st. stapes ; i.hy. interhyal ligament ; st.h. stylohyal 
cartilage ; h.h. hypohyal ; b.h. basibranchial ; th.h. rudiment 
of first branchial arch ; la. facial nerve. 

282 



404 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 

from the ventral part, becomes distinctly swollen, and 
clearly corresponds to the quadrate region of other types. 
The ventral part of the bar constitutes Meckel's carti- 
lage (mk). 

The hyoid arch has in the meantime become seg- 
mented into two parts, an upper part (i), which eventually 
becomes one of the small bones of the ear the incus- 
and a lower part which remains as the anterior cornu 
of the hyoid (st.h). The two parts continue to be con- 
nected by a ligament. 

The incus is articulated with the quadrate end of 
the mandibular arch, and its rounded head comes in 
contact with the stapes (Fig. 136, sf) which is segmented 
from the fenestra ovalis. 

According to some authors the stapes is independently formed 
from mesoblast cells surrounding a branch of the internal carotid 
artery. 

The main arch of the hyoid becomes divided into 
a hypohyal (hJi) below and a stylohyal (st.h) above, and 
also becomes articulated with the basal element of the 
arch behind (bh). 

In the course of further development the Meckelian 
part of the mandibular arch becomes enveloped in a 
superficial ossification forming the dentary. Its upper 
end, adjoining the quadrate region, becomes calcified 
and then absorbed, and its lower, with the exception of 
the extreme point, is ossified and subsequently incorpo- 
rated in the dentary. 

The quadrate region remains relatively stationary in 
growth as compared with the adjacent parts of the skull, 
and finally ossifies to form the malleiis. The processus 



XII.] THE AUDITORY OSSICLES. ' 405 

gracilis of the malleus is the primitive continuation into 
Meckel's cartilage. 

The malleus and incus are at first embedded in the 
connective tissue adjoining the tympanic cavity, which 
with the Eustachian tube is the persistent remains of 
the hyomandibular cleft ; and externally to them a bone 
known as the tympanic bone becomes developed so that 
they become placed between the tympanic bone and the 
periotic capsule. In late foetal life they become trans- 
ported completely within the tympanic cavity, though 
covered by a reflection of the tympanic mucous mem- 
brane. 

The dorsal end of the part of the hyoid separated 
from the incus becomes ossified as the tympano-hyal, 
and is anchylosed with the adjacent parts of the periotic 
capsule. The middle part of the bar just outside the 
skull forms the stylo-hyal (styloid process in man) which 
is attached by ligament to the anterior cornu of the 
hyoid (cerato-hyal). The tympanic membrane and ex- 
ternal auditory meatus develop as in the chick (p. 166). 

The ribs and sternum appear to develop in Mammals as in 
Birds (p. 234). 

The pectoral girdle, as in Birds (p. 234), arises as a con- 
tinuous plate of cartilage, the coracoid element of which is how- 
ever much reduced. 

The clavicle in Man is provided with a central axis of car- 
tilage, and its mode of ossification is intermediate between that of 
a true cartilage bone and a membrane bone. 

The pelvic girdle is formed in cartilage as in Birds, but in Man 
at any rate the pubic part of the cartilage is formed independently 
of the remainder. There are the usual three centres of ossification, 
which unite eventually into a single bone the innominate bone. 
The pubis and ischium of each side unite ventrally, so as com- 
pletely to enclose the obturator foramen. 



406 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 

The skeleton of the limbs develops so far as is known as in 
Birds, from a continuous mesoblastic blastema, within which the 
corresponding cartilaginous elements of the limbs become dif- 
ferentiated. 

The body cavity. The development of the body 
cavity and its subsequent division into pericardia! 
pleural and peritoneal cavities is precisely the same in 
Mammalia as in Aves (p. 264 et seq.). But in Mam- 
malia a further change takes place, in that by the for- 
mation of a vertical partition across the body cavity, 
known as the diaphragm, the pleural cavities, contain- 
ing the lungs, become isolated from the remainder of 
the body or peritoneal cavity. As shewn by their 
development the so-called pleurae or pleural sacs are 
simply the peritoneal linings of the anterior divisions 
of the body cavity, shut off from the remainder of the 
body cavity by the diaphragm. 

The vascular system. 

The heart. The two tubes out of which the heart 
is formed appear at the sides of the cephalic plates, 
opposite the region of the mid- and hind-brain (Fig. 
107). They arise at a time when the lateral folds 
which form the ventral wall of the throat are only just 
becoming visible. Each half of the heart originates in 
the same way as in the chick ; and the layer of the 
splanchnic mesoblast, which forms the muscular wall for 
each part (ahh). has at first the form of a half tube open 
below to the hypoblast. 

On the formation of the lateral folds of the splanchnic 
walls, the two halves of the heart become carried inwards 



XII.] ARTERIAL SYSTEM. 407 

and downwards, and eventually meet on the ventral 
side of the throat. For a short time they here remain 
distinct, but soon coalesce into a single tube. 

In Birds, it will be remembered, the heart at first has the 
form of two tubes, which however are in contact in front. It 
arises at a time when the formation of the throat is very much 
more advanced than in Mammalia ; when in fact the ventral 
wall of the throat is established as far back as the front end of 
the heart. 

In the lower types the heart does not appear till the ventral 
wall of the throat is completely established, and it has from the 
first the form of a single tub". 

It is therefore probable that the formation of the heart as two 
cavities is a secondary mode of development, which has been 
brought about by variations in the period of the closing in of the 
wall of the throat. 

The later development of the heart is in the main similar to 
that of the chick (p. 256 et seq.). 

The arterial system. The early stages of the 
arterial system of Mammalia are similar to those in 
Birds. Five arterial arches are formed, the three poste- 
rior of which wholly or in part persist in the adult. 

The bulbus arteriosus is divided into two (fig. 137 
B), but the left fourth arch (e), instead of, as in Birds, 
the right, is that continuous with the dorsal aorta, and 
the right fourth arch (i) is only continued into the right 
vertebral and right subclavian arteries. 

The fifth pair of arches which is continuous with 
one of the divisions of the bulbus arteriosus gives origin 
to the two pulmonary arteries. Both these however are 
derived from the arch on one side, viz. the left (fig. 137 
B); whereas in Birds, one pulmonary artery comes from 
the left and the other from the right fifth arch (fig. 
137 A). 



408 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 

The ductus Botalli of the fifth arch (known in Man 
as the ductus arteriosus) of the side on which the 
pulmonary arteries are formed, may remain (e.g. in Man) 
as a solid cord connecting the common stem of the 
pulmonary aorta with the systemic aorta. 

The diagram, Fig. 137, copied from Rathke, shews 
at a glance the character of the metamorphosis the 
arterial arches undergo in Birds and Mammals. 



FIG. 137. 





DIAGRAMS ILLUSTRATING THE METAMORPHOSIS OF THE AR- 
TERIAL ARCHES IN A BIRD A. AND A MAMMAL B. 

(From Mivart after Kathke.) 

A. a. internal carotid ; b. external carotid ; c. common carotid ; 
d. systemic aorta ; e. fourth arch of right side (root of dorsal 
aorta) / right subclavian ; g. dorsal aorta ; h. left subcla- 
vian (fourth arch of left side) ; i. pulmonary artery ; Jc. and 
I. right and left ductus Botalli of pulmonary arteries, 

B; a.' internal carotid ; b. external carotid ; c. common carotid ; 
d.' systemic aorta ; e. fourth arch of left side (root of dorsal 
aorta) ; /. dorsal aorta ; g. left vertebral artery ; h. left sub- 
clavian artery ; i. right subclavian (fourth arch of right 
side) ; Jc. right vertebral ; I. continuation of right subcla- 
vian ; m. pulmonary artery ; n. ductus Botalli of pulmonary 
artery. 



XII.] VENOUS SYSTEM. 409 

In some Mammals both subclavians spring from 
a trunk common to them and the carotids (arteria 
anonyma) ; or as in Man and some other Mammals, 
the left one arises from the systemic aorta just beyond 
the carotids. Various further modifications in the origin 
of the subclavians are found in Mammalia, but they 
need not be specified in detail. The vertebral arteries 
arise in close connection with the subclavians, whereas 
in Birds they arise from the common carotids. 

The venous system. In Mammals the same venous 
trunks are developed in the embryo as in Birds (Fig. 
138 A). The anterior cardinals or external jugulars 
form the primitive veins of the anterior part of the 
body, and the internal jugulars and anterior vertebrals 
are subsequently formed. The subclavians (Fig. 138 
A, s), developed on the formation of the anterior limbs, 
also pour their blood into these primitive trunks. In 
the lower Mammalia (Monotremata, Marsupialia, Insec- 
tivora, some Rodentia, etc.) the two ductus Cuvieri 
remain as the two superior venae cavse, but more usually 
an anastomosis arises between the right and left in- 
nominate veins, and eventually the whole of the blood 
of the left superior cava is carried to the right side, and 
there is left only a single superior cava (Fig. 138 B and 
C). A small rudiment of the Jeft superior cava remains 
however as the sinus coronarius and receives the coronary 
vein from the heart (Figs. 138 C, cor and 139 cs). 

The posterior cardinal veins form at first the only 
veins receiving the blood from the posterior part of the 
trunk and kidneys ; and on the development of the hind 
limbs receive the blood from them also. 

An unpaired vena cava inferior becomes eventually 



410 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 
FIG. 138. 





DIAGRAM OF THE DEVELOPMENT OF THE PAIRED VENOUS 
SYSTEM OF MAMMALS (MAN). (From Gegenbaur.) 

j. jugular vein ; cs. vena cava superior ; s. subclavian veins ; c. 
posterior cardinal vein ; v. vertebral vein ; az. azygos vein ; 
cor. coronary vein. 

A. Stage in which the cardinal veins have already disap- 
peared. Their position is indicated by dotted lines. 

B. Later stage when the blood from the left jugular vein is 
carried into the right to form the single vena cava superior ; a 
remnant of the left superior cava being however still left. 

C. Stage after the left vertebral vein has disappeared ; the 
right vertebral remaining as the azygos vein. The coronary vein 
remains as the last remnant of the left superior vena cava. 

developed, and gradually carries off a larger and larger 
portion of the blood originally returned by the posterior 
cardinals. It unites with the common stem of the 
allantoic and vitelline veins in front of the liver. 

At a later period a pair of trunks is established 
bringing the blood from the posterior part of the cardinal 
veins and the crural veins directly into the vena cava 



XII.] 



VERTEBRAL VEINS. 



411 



inferior (Fig. 139, il). These vessels, whose development 
has not been adequately investigated, form the common 




DIAGRAM OF THE CHIEF VENOUS TRUNKS OF MAN. 
(From Gegenbaur.) 

cs. coronary sinus ; s. subclavian vein ; ji. internal jugular ; 
Je. external jugular ; az. azygos vein ; ha. hemiazygos vein ; 

c. Jotted line shewing previous position of cardinal veins ; 

d. vena cava inferior ; r. renal veins ; il. iliac ; Ity. hypogas- 
tric veins ; h. hepatic veins. 

The dotted lines shew the position of embryonic vessels 
aborted in the adult. 

iliac veins, while the posterior ends of the cardinal veins 
which join them become the hypogastric veins (Fig. 
139 hy). 

Posterior vertebral veins, similar to those of Birds, 
are established in connection with the intercostal and 



412 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 

lumbar veins, and unite anteriorly with the front part 
of the posterior cardinal veins (Fig. 138 A). 

Upon the formation of the posterior vertebral veins, 
and upon the inferior vena cava becoming more im- 
portant, the middle part of the posterior cardinals be- 
comes completely aborted (Fig. 139 c), the anterior and 
posterior parts still persisting, the former as the con- 
tinuations of the posterior vertebrals into the anterior 
vena cava (az\ the latter as the hypogastric veins (%). 

Though in a few Mammalia both the posterior verte- 
brals persist, a transverse connection is usually established 
between them, and the one (the right), becoming the 
more important, constitutes the azygos vein (Fig. 139 
az\ the persisting part of the left forming the hemi- 
azygos vein (ha). 

The remainder of the venous system is formed in the 
embryo by the vitelline and allantoic veins, the former 
being eventually joined by the mesenteric vein so as to 
constitute the portal vein. 

The vitelline vein is the first part of this system 
established, and divides near the heart into two veins 
bringing back the blood from the yolk-sac (umbilical 
vesicle). The right vein soon however aborts. 

The allantoic (anterior abdominal) veins are origin- 
ally paired. They are developed very early, and at first 
course along the still widely open somatic walls of the 
body, and fall into the single vitelline trunk in front. 
The right allantoic vein disappears before long, and the 
common trunk formed by the junction of the vitelline 
and allantoic veins becomes considerably elongated. 
This trunk is soon envelop'ed by the liver, and later in 
its passage through, gives off branches to, and also 



XII.] SUPRA-RENAL BODIES. 413 

receives branches from this organ near its anterior exit. 
The main trunk is however never completely aborted, as 
in the embryos of other types, but remains as the ductus 
venosus Arantii. 

With the development of the placenta the allantoic 
vein becomes the main source of the ductus venosus, 
and the vitelline or portal vein, as it may perhaps be 
now conveniently called, ceases to join it directly, but 
falls into one of its branches in the liver. 

The vena cava inferior joins the continuation of the 
ductus venosus in front of the liver, and, as it becomes 
more important, it receives directly the hepatic veins 
which originally brought back blood into the ductus 
venosus. The ductus venosus becomes moreover merely 
a small branch of the vena cava. 

At the close of foetal life the allantoic vein becomes 
obliterated up to its place of entrance into the liver; 
the ductus venosus becomes a solid cord the so-called 
round ligament and the whole of the venous blood is 
brought to the liver by the portal vein. 

Owing to the allantoic (anterior abdominal) vein 
having merely a foetal existence an anastomosis between 
the iliac veins and the portal system by means of the 
anterior abdominal vein is not established. 

The supra-renal bodies. These are paired bodies 
lying anterior to the kidneys and are formed of two 
parts, (1) a cortical and (2) a medullary portion. They 
first appear in the Rabbit on the 12th or 13th day of 
gestation, and arise as masses of mesoblast cells lying 
between the aorta and the mesentery and to one side of 
the former. On the 14th day they are well marked, 
and lying dorsal to them is another mass of cells which 



414 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 

is found to be continuous with the sympathetic nervous 
system. 

On the 16th day processes from the sympathetic 
mass enter the mesoblastic tissue and become trans- 
formed into the medullary portion of the adult supra- 
renal ; while the mesoblastic tissue gives rise to the 
cortical layer, 

The urinogenital organs. 

The history of these organs in Mammalia, excepting 
so far as concerns the lower parts of the urinogenital 
ducts, is the same as in the Chick. 

The Wolffian body and duct first appear, and are 
followed by the Miillerian duct and the kidney. The 
exact method of development of the latter structures 
has not been followed so completely as in the Chick; 
and it is not known whether the peculiar structures 
found. at the anterior end of the commencing Miillerian 
duct in Aves occur in Mammalia. 

The history of the generative glands is essentially 
the same as in the Chick. 

Outgrowths from a certain number of Malpighian 
bodies in the Wolffian body are developed along the 
base of the testis, and enter into connection with the 
seminiferous stroma. It is not certain to what parts of 
the testicular tubuli they give rise, but they probably 
form at any rate the vasa recta and rete vasculosum. 
Similarly intrusions from the Malpighian bodies make 
their way into the ovary of the female, and give rise to 
cords of tissue which may persist throughout life. 

The vasa efferentia (coni vasculosi) appear to be 
derived from the glandular tubes of part of the Wolffian 



XII.] GENITAL CORD. 415 

body. The Wolffian duct itself becomes in the male the 
vas deferens and the convoluted canal of the epididy- 
mis ; the latter structure except the head being entirely 
derived from the Wolffian duct. 

The functionless remains of the embryonic organs described 
for the chick (p. 224) are found also in mammals. 

The Miillerian ducts persist in the female as the 
Fallopian tubes and uterus. 

The lower parts of the urinogenital ducts are some- 
what further modified in the Mammalia than the Chick. 

The genital cord. The lower part of the Wolffian 
ducts becomes enveloped in both sexes in a special cord 
of tissue, known as the genital cord (Fig. 1 40 gc), within 
the lower part of which the Mulleriaii ducts are also 
enclosed. In the male the Miillerian ducts in this cord 
atrophy, except at their .distal end where they unite to 
form the uterus masculinus. The Wolffian ducts, after 
becoming the vasa deferentia, remain for some time 
enclosed in the common cord but afterwards separate 
from each other. The seminal vesicles are outgrowths of 
the vasa deferentia. 

In the female the Wolffian ducts within the genital 
cord atrophy, though rudiments of them are for a long 
time visible or even permanently persistent. The lower 
parts of the Miillerian ducts unite to form the vagina 
and body of the uterus while the upper become the 
horns of the uterus and the Fallopian tubes. The 
junction commences in the middle and extends forwards 
and backwards ; the stage with a median junction being 
retained permanently in Marsupials. 

The urinogenital sinus and external generative 
organs. The dorsal part of the cloaca with the alimen- 



416 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 

tary tract becomes partially constricted off from the 

ventral, which then forms a urinogenital sinus (Fig. 140 

ug). In the course of development the urinogenital 

FIG. 140. 




DIAGRAM OF THE URINOGENITAL ORGANS OF A MAMMAL AT 
AN EARLY STAGE. (After Allen Thomson ; from Quain's 
Anatomy.) 
The parts are seen chiefly in profile, but the Mlillerian and 

Wolffian ducts are seen from the front. 

3. ureter; 4. urinary bladder; 5. urachus ; ot. genital ridge 
(ovary or testis) ; W. left Wolffian body ; x. part at apex 
from which coni vasculosi are afterwards developed ; w. 
Wolffian duct ; m. Miillerian duct ; gc. genital cord consist- 
ing of Wolffian and Miillerian ducts bound up in a common 
sheath ; i. rectum ; ug. urinogenital sinus ; cp. elevation 
which becomes the clitoris or penis ; Is. ridge from which the 
labia majora or scrotum are developed. 



XII.] EXTERNAL GENERATIVE ORGANS. 417 

sinus becomes, in all Mammalia but the Ornithodelphia, 
completely separated from the intestinal cloaca, and the 
two parts obtain separate external openings. The 
ureters (Fig. 140, 3) open higher up than the other 
ducts into the stalk of the allantois which here dilates 
to form the bladder. That part of the stalk which con- 
nects the bladder with the ventral wall of the body 
constitutes the urachus, and loses its lumen before the 
close of embryonic life. The part of the stalk of the 
allantois below the openings of the ureters narrows to 
form the urethra, which opens together with the Wolffi an 
and Mullerian ducts into the urogenital cloaca. 

In front of the urogenital cloaca there is formed 
a genital prominence (Fig. 140 cp) with a groove con- 
tinued from the urinogenital opening, and on each side a 
genital fold (Is). In the male the sides of the groove on 
the prominence coalesce together, embracing between 
them the opening of the urinogenital cloaca, and the 
prominence itself gives rise to the penis, along which the 
common urinogenital passage is continued. The two 
genital folds unite from behind forwards to form the 
scrotum. 

In the female the groove on the genital prominence 
gradually disappears, and the prominence remains as the 
clitoris, which is therefore the homologue of the penis : 
the two genital folds form the labia majora. The urethra 
and vagina open independently into the common uro- 
genital sinus. 

THE ALIMENTARY CANAL AND ITS APPENDAGES. 

It is convenient to introduce into our account of the 
organs derived from the hypoblast, a short account of 
F. & B. 27 



418 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 

certain organs connected with the alimentary canal 
such as the mesentery, stomodaeum, etc., which are not 
hypoblastic in origin. 

The origin of the hypoblast, and the process of 
folding by which the cavity of the mesenteron is 
established have already been described. The mesen- 
teron may be considered under three heads. 

1. The anterior or respiratory division of the 
mesenteron. The pharynx, thyroid body, Eustachian 
tube, tympanic cavity, oesophagus, trachea, bronchi, lungs 
and stomach are developed from this portion, and their 
development in the Mammal so closely resembles that in 
the Chick that it is unnecessary for us to add to the 
account we have already given in the earlier part of this 
work. 

This section of the alimentary canal, as in the Chick, 
is distinguished in the embryo by the fact that its walls 
send out a series of paired diverticula which meet the 
skin, and, after perforation has been effected at the 
regions of contact, form the visceral clefts. 

2. The middle division of the mesenteron, from 
which the liver and pancreas are developed, as in the 
Chick, forms the intestinal and cloacal region and is at 
first a straight tube. It remains for some time connected 
with the yolk sack. 

The Cloaca appears as a dilatation of the mesen- 
teron which receives, as in Aves, the opening of the 
allantois almost as soon as the posterior section of 
the alimentary tract is established. The eventual 
changes which it undergoes have already been dealt 
with in connection with the urinogenital organs. 

The intestine. The posterior part of this becomes 



XII.] THE MESENTERY. 419 

enlarged to form the large intestine, while the anterior 
portion becoming very much elongated and coiled forms 
the small intestine, and moreover gives rise anteriorly 
to the liver and pancreas. 

From the large intestine close to its junction with the small 
intestine an outgrowth is developed, the proximal part of which 
enlarges to form the ccecum, while the distal portion in Man 
forms the vermiform appendix. 

3. The postanal division of the mesenteron atro- 
phies at an early period of embryonic life. In the Chick 
and lower types it communicates for a short time with 
the hind end of the neural canal. 

Splanchnic mesoblast and mesentery. The mesen- 
teron consists at first of a simple hypoblastic tube, which 
however becomes enveloped by a layer of splanchnic 
mesoblast. This layer, which is not at first continued 
over the dorsal side of the mesenteron, gradually grows 
in, and interposes itself between the hypoblast of the 
mesenteron, and the organs above. At the same time 
it becomes differentiated into two layers, viz. an outer 
epithelioid layer which gives rise to part of the peritoneal 
epithelium, and an inner layer of undifferentiated cells 
which in time becomes converted into the connective 
tissue and muscular walls of the mesenteron. The 
connective tissue layers are first formed, while of the 
muscular layers the circular is the first to make its 
appearance. 

Coincidently with the differentiation of these layers 
the connective tissue stratum of the peritoneum becomes 
established. 

The mesentery is developed as in the Chick (p. 172). 
In the thoracic region it is hardly if at all developed. 

272 



420 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 

The primitive simplicity in the arrangement of the 
mesentery is usually afterwards replaced by a more com- 
plicated disposition, owing to the subsequent elongation 
and consequent convolution of the intestine and stomach. 

The layer of peritoneal epithelium on the ventral 
side of the stomach is continued over the liver, and 
after embracing the liver, becomes attached to the 
ventral abdominal wall. Thus in the region of the liver 
the body-cavity is divided into two halves by a mem- 
brane, the two sides of which are covered by the peri- 
toneal epithelium, and which encloses the stomach 
dorsally and the liver ventrally. The part of the mem- 
brane between the stomach and liver is narrow, and 
constitutes a kind of mesentery suspending the liver 
from the stomach : it is known to human anatomists as 
the lesser omentum. 

The part of the membrane connecting the liver with 
the anterior abdominal wall constitutes the falciform or 
suspensory ligament of the liver. It arises by a secondary 
fusion, and is not a remnant of a primitive ventral 
mesentery (vide p. 264). 

The mesentery of the stomach, or mesogastrium, 
enlarges in Mammalia to form a peculiar sack known as 
the greater omentum. 

The stomodseum. The anterior section of the per- 
manent alimentary tract is formed, as in the Chick, by 
an invagination of epiblast, constituting a more or less 
considerable pit, with its inner wall in contact with the 
blind anterior extremity of the mesenteron. 

From the epiblastic liniog of this pit are developed 
the pituitary body and the salivary as well as the other 
buccal glands. 



XII.] THE TEETH. 421 

FIG. 141. 




DIAGRAM SHEWING THE DIVISION OF THE PRIMITIVE BUCCAL 
CAVITY INTO THE RESPIRATORY SECTION ABOVE AND THE 
TRUE MOUTH BELOW. (From Gegenbaur.) 

p. palatine plate of superior maxillary process; m. permanent 
mouth; n. posterior part of nasal passage; e. internasal 
septum. 

A palate grows inwards from each of the superior 
maxillary processes (Fig. 141), which, meeting in the 
middle line, form a horizontal septum dividing the front 
part of the stomodaeum into a dorsal respiratory section, 
containing the opening of the posterior nares, and a 
ventral cavity forming the permanent mouth. These 
two divisions open into a common cavity behind. This 
septum on the development within it of an osseous 
plate constitutes the hard palate. A posterior pro- 
longation in which no osseous plate is formed consti- 
tutes the soft palate. An internasal septum (Fig. 141 e) 
may more or less completely divide the dorsal cavity 
into two canals, continuous respectively with the two 
nasal cavities. 

The teeth are special products of the oral mucous 
membrane. They are formed from two distinct organs, 
viz. an epithelial cap and a connective tissue papilla, 



422 DEVELOPMENT OF ORGANS IN MAMMALIA. [XII. 

which according to most authors give rise to the enamel 
and dentine respectively. 

The proctodsBUm. The cloacal section of the ali- 
mentary canal is placed in communication with the 
exterior by means of a shallow epiblastic invagination 
constituting the proctodseum. 



APPENDIX. 



PRACTICAL INSTRUCTIONS FOB STUDYING THE DE- 
VELOPMENT OF THE CHICK. 

I. A. Incubators. 

OF all incubators, the natural one, i.e. the hen, 
is in some respects the best. The number of eggs 
which fail to develope is fewer than with an arti- 
ficial incubator, and the development of monstrosi- 
ties is rarer. A good sitter will continue to sit 
for thirty or more days at least, even though the 
eggs are daily being changed. She should never 
be allowed to want for water, and should be well 
supplied according to her appetite with soft food. 
It is best to place the food at some little distance 
from the eggs, in order that the hen may leave 
the eggs when feeding. She will sit most per- 
sistently in a warm, quiet, somewhat darkened 
spot. When an egg is placed under her, the date 
should be marked on it, in order that the duration 
of its incubation may be exactly known. When 
the egg is intended to remain for some time, e.g. 
for seven days or more, the mark should be bold 
and distinct, otherwise it will be rubbed off. 



424 PRACTICAL DIRECTIONS. [APR 

On the whole however we have found it more 
convenient to use a good artificial incubator. We 
have ourselves used with success two different 
incubators. One made by the Cambridge Scientific 
Instrument Company, and the other by Wiesnegg 
of 64, Rue Gay-Lussac, Paris (Fig. 65 in his 
catalogue for 1881). We have had the longest ex- 
perience with the former, and have found it work 
exceedingly well : having been able to hatch chicks 
without more attention than now and then turning 
over the eggs. 

Both these incubators consist essentially of a 
large water-bath fitted with a gas regulator. They 
are both perfectly automatic and when once regu- 
lated require no further attention. 

The temperature within the incubator should 
be maintained at from 37 to 40C. A rise above 
40 is fatal ; but it may be allowed to descend to 
35 or in the young stages lower, without doing 
any further harm than to delay the development. 

The products of the combustion of the gas 
should be kept as much as possible from the eggs, 
while ou supply of fresh air and of moisture is 
essential. 

Tolerably satisfactory results may be obtained with 
an ordinary chemical double- jacketed drying water-bath, 
thoroughly covered in with a thick coat of cotton wool 
and flannel baize, and heated by a very small gas-jet. 
If the vessel be filled with hot water, and allowed to cool 
down to 40 or thereabouts, before the eggs are introduced, 
a very small gas flame will be sufficient to maintain the 
requisite temperature. A small pin-hole-nozzle, giving 
with ordinary pressure an exceeding narrow jet of flame 
about two inches high, is the most convenient. By turn- 
ing the gas off or on, so as to reduce or increase the height 



APP.] HARDENING EMBRYOS. 425 

of the jet as required, a very steady mean temperature 
may be maintained. 

In the absence of gas, a patent night-light placed at a 
proper distance below the bath may be made to answer 
very well. When a body of water, once raised to the 
necessary temperature, is thoroughly surrounded with 
non-conducting material, a very slight constant amount of 
heat will supply all the loss. 



B. On preparing sections of the embryo. 
1. HARDENING. 
a. Picric acid. 

We find this reagent the most satisfactory 
for hardening the chick and in most instances 
mammalian embryos. 

Klein enberg's solution of picric acid is the 
best. 

With 100 parts of water, make a cold 
saturated solution of picric acid ; add to this 
two parts of concentrated sulphuric acid or 
nitric acid : filter and add to the filtrate three 
times its bulk of water. 

In this solution of picric acid 1 the embryo 
must be placed and left for from 2 5 hours. 
It should then be washed in alcohol of 30 p.c. 
and placed in alcohol 50 p.c. for one hour. 
From this it must be removed into alcohol 
of 70 p.c. in which it should be left until 
all the picric acid is extracted ; to facilitate 
this the 70 p.c. alcohol should be frequently 
changed : when free from picric the embryo 

1 It is sometimes advantageous to add to this solution of picric 
acid as much pure kreasote as it will dissolve (vide Kleinenberg, 
'Development of Earthworm," Quarterly Journal of Mic. Sci. 1879). 



426 PRACTICAL DIRECTIONS. [APP. 

should be placed in 90 p.c. alcohol and kept 
there until required for further use. 

2sT.B. Hardened embryos should always be 
kept in 90 p.c. spirit and only placed in abso- 
lute before imbedding, or staining with haema- 
toxylin. 

Some histologists prefer to keep hardened tissues 
in alcohol 70 p.c. 

b. Corrosive sublimate. 

Place the embryo in a large quantity of a 
saturated aqueous solution of corrosive subli- 
mate to which a few drops of glacial acetic acid 
have been added, and allow it to remain for 
half-an-hour 1 . It is necessary thoroughly to ex- 
tract the corrosive sublimate from the cells of the 
embryo ; to accomplish this, wash it thoroughly 
with water for from 10 minutes to 3 hours ac- 
cording to the size of the object. The washing 
may be limited to frequent changes of water or 
the embryo may be placed in a vessel through 
which a continuous stream of water is kept 
running. "When all the sublimate is removed, 
place it in 50 p.c. alcohol acidulated with nitric 
acid (half-a-dozen drops of acid to a 4 oz. 
bottle of spirit) for five minutes. The preser- 
vation of the embryo is completed by treating 
it with 70 p.c. alcohol for twenty-four hours and 
then keeping it in 90 p.c. alcohol. We have 
not found that corrosive sublimate gives such 
good results as picric acid in the case of chicks 
and mammalian embryos. 

1 If there is only a small quantity of acetic acid mixed with the 
sublimate, a prolonged immersion will do the embryo no harm. 



APP.] HARDENING EMBRYOS. 427 

c. Osmic acid. 

Osmic acid is a difficult reagent to use, but 
when properly applied it gives most excellent 
results. 

It should be used as a weak solution ('I to 
5 p.c.). The object should be left in it until 
it has acquired a light brown tint. The stronger 
the solution the less time is required for the 
production of this tint. It should then be 
removed and placed in picro- carmine, which 
arrests the action of the osmic and stains the 
embryo. The time required for the picro-car- 
mine staining must be determined by practice. 
From the picro-carmine the object must be 
washed in 70 p.c. spirit; and then placed in 
90, or may be preserved directly in glycerine. 

If it is desired to use other staining agents 
(borax-carmine is good for some preparations), 
the object must be removed from osmic into 
water or weak spirit, thence through 50 into 
70 p.c., stained, and passed through 70 to 
90 p.c. spirit. 

d. After using osmic it is well in some cases 

(mammalian segmenting ova) to place the 
object in Miiller's fluid for 2 or 3 days, after 
which it may be preserved in glycerine or spirit. 
Miiller's fluid is made by dissolving 25 grms. 
of bichromate of potash and 10 grms. of sodic 
sulphate in 1000 cc. of water. 

e. With chromic acid. 

The embryo must be immersed in a solution 
of the strength of *1 p.c. for 24 hours. From 
this it should be removed and placed in a stronger 



428 PRACTICAL DIRECTIONS. [APP. 

solution (-3 p.c.) for another 24 hours. If it 
then appears sufficiently hard, it may be at 
once placed in alcohol of 70 p.c., in which it 
should remain for one day, and then be trans- 
ferred to alcohol of 90 p.c. 

f. Absolute alcohol has also been employed as 

a hardening reagent, but is by no means so good 
as the reagents recommended above. 

The object of these so-called hardening reagents is 
to kill the tissues with the greatest possible rapidity 
without thereby destroying them. The subsequent 
treatment with alcohol completes the hardening which 
is only commenced by these reagents. 

There is room for the exercise of considerable skill 
in the use of alcohol, and this skill can only be acquired 
by experience. A few general rules may however be 
laid down. 

(1) Tissues should not, generally, be changed from water 
or an aqueous solution of the first hardening reagent 
into an alcoholic solution of too great strength, nor 
should the successive solutions of alcohol used differ 
too much in strength. The distortion produced by 
the violence and inequality of the diffusion currents 
is thus diminished. This general rule should be 
remembered in transferring tissues from alcohol to 
the staining agents and vice versa. 

(2) The tissues should not be left too long (more than 
one or two hours) in alcoholic solutions containing 
less than 70 p.c. of alcohol. 

(3) They should not be kept in absolute alcohol longer 
than is necessary to dehydrate them (see B. 1, p. 426). 
The alcoholic solutions we generally use contain 30, 
50, 70, 90 p.c. of alcohol. 

2. STAINING. 

In most cases it will be found of advantage 
to stain the embryo. The best method of doing 



APR] STAINING EMBRYOS. 429 

this is to stain the embryo as a whole, rather 
than to stain the individual sections after they 
have been cut. 

We have found hsematoxylin and borax- 
carmine the best reagents for staining embryos 
as a whole. 

a. With hsematoxylin. 

The best solution of hsematoxylin, one for 
which we are indebted to Kleinenberg, is made 
in the following way. 

(1) Make a saturated solution of crystallized cal- 
cium chloride in 70 p. a alcohol, and add 
alum to saturation. 

(2) Make also a saturated solution of alum in 70 
p.c. alcohol, and add 1 to 2 in the proportion 
of 1 : 8. 

(3) To the mixture of 1 and 2 add a few drops of 
a saturated solution of hsematoxylin in ab- 
solute alcohol. 

(4) It is often the case that hsematoxylin solution 
prepared in this way has not the proper 
purple tint ; but a red tint. This is due to 
acidity of the materials used. The proper 
colour can be obtained by treating it with 
some alkaline solution. "We have found it 
convenient to use for this purpose a saturated 
solution of sodium bi-carbonate in 70 p.c. 
spirit. (The exact amount must be deter- 
mined by experiment, as it depends upon the 
amount of acid present.) 

The embryo should be placed for some hours 
in absolute alcohol, before staining with hse- 



430 PRACTICAL DIRECTIONS. [APP. 

matoxylin, and should be removed directly from 
absolute into the haematoxylin. 

The time required for staining varies with 
the size of the object and the strength of the 
staining fluid. Hsematoxylin will not stain if 
the embryo is not quite free from acid. 

If the embryo is stained too dark, it should 
be treated with a solution of 70 p.c. alcohol 
acidulated with nitric acid (*25 p.c. of acid) 
until the excess of staining is removed; and in 
all cases the hsematoxylin staining is improved 
by treating the embryo with acidulated 70 p.c. 
alcohol. 

After staining the embryo must be well 
washed in 70 and placed in 90 p.c. spirit. 

b. With borax-carmine. 

Make an aqueous solution of 2 to 3 p.c. 
carmine and 4 p.c. borax, by heating: add an 
equal volume of 70 p.c. alcohol, and let the 
mixture stand for thirty-six hours; after which 
carefully filter. 

Stain the object thoroughly by leaving it in 
this solution for one or even two days; it will 
attain a dull maroon colour : transfer it then to 
acidulated alcohol (see a) until it becomes a 
bright red, and afterwards keep it as before in 
90 p.c. alcohol. 

This staining solution permeates more tho- 
roughly and uniformly a large object than does 
hsematoxylin : therefore when a four or five day 
chick is to be stained, borax-carmine is the best 
staining reagent to use. Embryos that have 
been preserved in corrosive sublimate will be 



APP.] STAINING EMBRYOS. 431 

found to stain more thoroughly in this than in 
the hsematoxylin solution. 

c. With carmine. 

Beale's carmine or some alcoholic solution is 
the best. Into this the embryo may be removed 
directly from 90 p.c. alcohol, left for 24 hours, 
and then placed again in alcohol until required. 

d. With picro-carmine. 

This reagent is useful as will be seen later 
for staining mammalian segmenting ova and 
very young blastoderms ; it is used with the 
greatest success after hardening in osmic acid. 

There are several methods of making picro- 
carmiue, the following is the simplest, and we 
have found it answer our purpose fairly well. 

To a solution made up of 1 grm. of car- 
mine 4 cc. of liquor ammonia and 200 cc. of 
distilled water add 5 grms. of picric acid; agitate 
the mixture for some minutes, and then decant, 
leaving the excess of acid. 

The decanted fluid must remain for several 
days, being stirred up from time to time; even- 
tually evaporated to dryness in a shallow vessel, 
and to every 2 grms. of the residue add 100 cc. 
of distilled water. 

e. With alum carmine. 

To make it, boil a strong aqueous solution of 
ammonia-alum with excess of carmine for 10 to 
20 minutes, filter, and dilute the filtrate until 
it contains from 1 to 5 p.c. of alum. Add a 
few drops of carbolic acid to prevent the growth 
of fungus. 






432 PRACTICAL DIRECTIONS. [APR 

Well hardened tissues may be left in this 
aqueous solution for 24 hours. It is especially 
good for staining nuclei ; as a rule the staining 
is not diffuse, but it is necessary after staining 
to treat with acid alcohol (see a). 

3. IMBEDDING AND CUTTING SECTIONS. 

It is not possible to obtain satisfactory sec- 
tions of embryos without employing some 
method of imbedding, and using a microtome. 
Many imbedding solutions and methods of cut- 
ting sections have been used, but we find the 
following far superior to any other. It combines 
several advantages \ in the first place it renders 
it comparatively easy to obtain, what is so 
essential, a complete consecutive series of sec- 
tions of the embryo ; and secondly, all the sec- 
tions when mounted are in the same relative 
position ; and the various parts of each section 
retain their normal position with regard to 
each other. 

a. Imbedding. 

The substance we prefer for imbedding is 
paraffin. As will be seen below it is necessary 
to have at hand paraffins of various melting 
points, according to the temperature of the 
room at the time when the sections are cut. 

It will be found most convenient to obtain 
paraffins of the highest and lowest melting 
points and to mix them together as experience 
dictates. 

Place the stained embryo in absolute alco- 
hol until completely dehydrated (two hours is 
sufficient for small embryos) : and when ready 



APP.] IMBEDDING. 433 

to imbed soak it in turpentine 1 until it is com- 
pletely saturated : and transfer it thence with as 
little turpentine as possible to a dish of melted 
paraffin. 

In cases of very delicate tissues, it is better to use 
chloroform instead of turpentine. The chloroform 
should be carefully added by means of a pipette to the 
absolute alcohol in which the tissue is placed. The 
chloroform sinks to the bottom of the bottle or tube 
and the embryo, which at first lies at the junction of the 
two liquids, gradually sinks into the chloroform. When 
this is accomplished, remove all the absolute with a 
pipette and add pieces of solid paraffin to the chloroform. 
Gently warm this on a water bath till all the chloroform 
is driven off ; then imbed in the usual way. 

Care must be taken that no more heat is 
used than is necessary to melt the paraffin ; for 
this purpose the paraffin should be warmed over 
a water bath the temperature of which is kept 
constant (from 50 to 60C. but not more than 
60C.). 

A paraffin melting at 44C. is of the proper consistency 
for cutting when the temperature of the room is 15C. 



With care a porcelain evaporating dish and 
a gas flame may be made to answer, but the 
student is advised not to imbed without a 
water bath. 

The embryo may be left in the paraffin two, 
three or more hours, after which it is imbedded 
by placing it along with the melted paraffin in 
either a box. made by bending up the sides and 
folding in the corners of a piece of stiff" paper, 
or what is better, a box formed by two L-shaped 

1 If the alcohol is not quite absolute kreasote should be used 
instead of turpentine. 

F. & B. 28 



PRACTICAL DIRECTIONS, [APP. 

pieces of lead, placed on a glass slide in such a 
manner as to enclose a space, The latter is 
preferable because the object can be placed 
in any position required with great ease by 
moving it with a hot needle, and the whole can 
be cooled rapidly. It is advisable, at any rate at 
first, to arrange the embryo so as to cut it into 
transverse sections. 

When cool a block of paraffin is formed, in 
the midst of which is the embryo. 

Other imbedding agents have been used. The best 
of these are, (1) pure cocoa butter ; (2) a mixture of 
spermaceti and castor oil or cocoa butter (4 parts of 
the former to one of the latter). With these imbedding 
substances, it is generally necessary to moisten the razor, 
either with olive oil or turpentine and ribbons of sec- 
tions cannot be made (see b). 

Cutting sections. 

When the imbedding block is cold pare away 
the edges, then gradually slice it away until the 
end of the embryo is near the surface, and 
place it in a microtome. 

The microtome we are most accustomed to is 
a ' sliding microtome' made by Jung of Heidel- 
berg ; it gives excellent results. Recently how- 
ever Messrs CaldweH and Threlfall have designed 
an automatic microtome which has been used 
with success at the Cambridge Morphological 
Laboratory and promises to effect a great saving 
of time and trouble in cutting sections (vide p. 471 
and Proceedings of the Cambridge Phil. Soc. 1883). 
A convenient small microtome is one made by 
Zeiss of Jena (also by the Cambridge Scientific 
Instrument Company), in which the object is 
fixed and by means of a finely divided screw 



APP.] CUTTING SECTIONS. 435 

raised through a hole in a glass plate, across 
which a razor held in the hand is pushed. We 
will briefly describe the method of manipulation 
for the small microtome, it will be found easily 
applicable to Jung's sliding microtome. 

The paraffin block is pared in such a manner 
that the edge nearest to the operator and that 
opposite to him are parallel. A dry razor is 
then pushed upon the glass plate over the hole 
through which the block of paraffin projects up- 
wards, and a section cut which remains upon 
the razor. Care must be taken that the edge of 
the razor is parallel to the parallel edges of the 
paraffin block. The block having been raised 
by the screw, a second section is made in the 
same way and on the same part of the razor as 
the first ; in consequence of which, the first 
section will be pushed backwards by the second. 
Similarly each new section pushes backwards 
those already made ; and a ribbon of sections 
formed which, if the paraffin is of the right 
consistency, will adhere firmly together. 

Experience must teach the manipulator how 
to mix the paraffin in such a manner that it is 
neither too hard nor too soft ; if it is too hard, 
the sections will not adhere together and will 
curl up on the razor, if too soft they will 
stick to the razor and be found to be creased. 
When it is not possible to keep the temperature 
of the room constant it will be found convenient 
to use a hard paraffin, and when necessary to 
raise the temperature by means of a lamp. 

The paraffin should completely surround the 
embryo and fill up all the spaces within it. 

282 



436 PRACTICAL DIRECTIONS. [APP. 

c. Mounting sections. 

When the sections are cut, place them in 
rows on a slide prepared in the following manner. 
Make a solution of white shellac in kreasote 
by heating, and let it be of the consistency of 
glycerine, or slightly more fluid. With a camel's 
hair-brush paint a very thin and uniform layer 
of this gum over the slide which must be clean 
and dry, and while the gum is wet place the sec- 
tions in rows upon it. Now place the slide on a 
water bath which is heated up to the melting 
point of the paraffin. The sections sink down 
into the thin layer of shellac and kreasote, the 
kreasote slowly evaporates and the shellac be- 
coming hard fixes the section in the position in 
which it was placed on the slide. When the 
kreasote has been evaporated, pour turpentine 
carefully upon the slide, this dissolves the pa- 
raffin and clears the sections which may at once 
be mounted in Canada balsam. 

A turpentine or chloroform solution of Canada balsam 
should be used. 

This method of cutting ribbons of sections 
was first introduced by Mr Caldwell, to whom 
we are also indebted for the account given above 
for mounting sections (vide Note B, p. 471). 
The latter however is a modification and im- 
provement of Dr Giesbrecht's method. (Zoolo- 
gischer Anzeiger No. 92, 1881.) 

C. Preservation of the embryo as a whole. 

Chick embryos of the first or second day may be 
easily preserved whole as microscopic objects. For 
this purpose, the embryo, which has been preserved 






APP.] OPENING THE EGG. 487 

in the ordinary way (B, a) should be stained slightly, 
dehydrated, soaked in oil of cloves until transparent 
and mounted in balsam. 

Whole embryos of a later date cannot be satis- 
factorily preserved as microscopic objects. 

PRACTICAL DIRECTIONS FOR OBTAINING AND STUDYING 
CHICK EMBRYOS. 

II. Examination of a 36 to 48 hours 1 embryo. 

The student will find it by far the best plan to begin 
with the study of an embryo of this date. The manipu- 
lation is not difficult ; and the details of structure are 
sufficiently simple to allow them to be readily grasped. 
Earlier embryos are troublesome to manage until some 
experience has been gained; and the details of later 
ones are so many as to render it undesirable to begin 
with them. 

A. Opening tlie Egg. 

Take the egg warm from the hen or the incu- 
bator, and place it (it does not matter in what posi- 
tion, since the blastoderm will at this stage always 
be found at the uppermost part of the egg) in a 
small basin large enough to allow the egg to be 
covered with fluid. It is of advantage, but not 
necessary, to place at the bottom of the basin a 
mould, e.g. a flat piece of lead with a concavity on 
the upper surface, in which the egg may rest securely 
without rolling. Pour into the basin so much of a 
'75 per cent, solution of sodium chloride warmed to 
38C. as will cover the egg completely. With a sharp 
tap break through the shell at the broad end over 
the air-chamber, and let out as much air as has 
already been gathered there. Unless this is done, 



438 PRACTICAL DIRECTIONS. [APP. 

the presence of air in the air-chamber will cause the 
broad end to tilt up. At this date there will be 
very little air, but in eggs of longer incubation, in- 
convenience will be felt unless this plan be adopted. 

Instead of being broken with a blow, the shell 
may be filed through at one point, and the opening 
enlarged with the forceps; but a little practice will 
enable the student to use the former and easier- 
method without doing damage. 

With a blunt pair of forceps, remove the shell 
carefully bit by bit, leaving the shell-membrane 
behind; begin at the hole made at the broad end, 
and work over the upper part until about a third or 
half of the shell has been removed. 

Then with a finer pair of forceps remove the 
shell-membrane; it will readily come away in strips, 
torn across the long axis of the egg in a somewhat 
spiral fashion. The yolk and embryo will now come 
into view. 

It is the practice of some simply to break the egg 
across and pour the yolk and white together into a 
basin, very much as the housewife does. We feel 
sure, however, that the extra trouble of the method 
we have given will be more than repaid by the 
results. 

During this time, and indeed during the whole 
period of the examination of the embryo in situ, the 
basin and its contents must be maintained, either by 
renewal of the salt solution, or by the basin being 
placed on a sand-bath, at about 38C. 

B. Examination of the blastoderm in situ. 

This may be done with the naked eye, or with a 
simple lens of low power. Observe : 



APP.] REMOVAL OF THE EMBRYO. 439 

1. Lying across the long axis of the egg, the pellucid 
area, in the middle of which the embryo may be 
obscurely seen as a white streak. 

2. The mottled vascular area, with the blood-vessels 
just beginning to be formed. 

3. The opaque area spreading over the yolk with the 
changes in the yolk around its periphery. 

4. (With a simple lens), the contractions of the heart; 
perhaps the outlines of the head of the embryo 
may be detected. 

C. Removal of the embryo. 

Plunge one blade of a sharp fine pair of scissors 
through the blastoderm, just outside the outer margin 
of the vascular area, and rapidly carry the incision 
completely round until the circle is complete, avoid 
as much as possible any agitation of the liquid in the 
basin. 

With a little trouble, the excised blastoderm may 
now be floated into a watch-glass, care being taken to 
keep it as flat as possible. With a pair of forceps or 
with a needle, aided by gentle shaking, remove the 
piece of vitelline membrane covering the blastoderm. 

If any yolk adheres to the blastoderm, it may with 
a little gentle agitation easily be washed off. Some- 
times it is of advantage to suck up the yolk with a 
glass syringe, replacing the fluid removed with clean 
('75 p.c.) salt solution. 

The blastoderm should now be removed from the 
watch-glass to a microscopic glass slide ; since it is 
difficult in the former to prevent the edges of the 
blastoderm from curling up. 



440 PRACTICAL DIRECTIONS. [APP. 

The transference may easily be effected, if both 
the watch-glass and slide are plunged into a basin of 
clean warm salt solution. With a little care, the 
blastoderm can then be floated from the one to the 
other, and the glass slide, having the blastoderm with 
its upper surface uppermost spread flat upon it, very 
gently raised out of the liquid. 

A thin ring of putty may now be placed round 
the blastoderm, a small quantity of salt solution 
gently poured within the ring, and the whole covered 
with a glass slide, which may be pressed down until 
it is sufficiently close to the embryo. The presence 
of any air-bubbles must of course be avoided. 

Provided care be otherwise taken to keep the 
embryo well covered with liquid, the putty ring and 
the coverslip may be dispensed with. They are often 
inconvenient, as when the embryo has to be turned 
upside down. 

The object is now re&dy for examination with a 
simple lens or with a compound microscope of low 
objective. It is by far the best for the student to 
begin at least with the simple lens. In order that 
everything may be seen at its best, the slide should 
be kept warmed to about 38, by being placed on a 
hot stage. 

D. Surface view of the transparent embryo 
from above. 

The chief points to be observed are : 

1. The head-fold. 

2. The indications of the amnion; especially the 
false amnion, or outer amniotic fold. 






APP.] SURFACE VIEW. 441 

3. The neural tube : the line of coalescence of the 
medullary folds, the first cerebral vesicle, the com- 
mencing optic vesicles, the indications of the 
second and third cerebral vesicles, the as yet open 
medullary folds at the tail end. 

4. The heart seen dimly through the neural tube; note 
its pulsation if present. 

5. The fold of the somatopleure anterior to the heart 
(generally very faintly shewn). 

G. The fold of the splanchnopleurt (more distinctly 
seen) : the vitelline veins. 

7. The mesoblastic somites. 

8. Indications of the vitelline arteries. 

9. The as yet barely formed tail-fold. 

10. The commencing blood-vessels in the pellucid and 
vascular areas. 

E. Surface view of the transparent embryo from 
below. 

The coverslip must now be removed and the glass 
slide again immersed in a vessel of clean salt solu- 
tion. By gently seizing the extreme edge of the 
opaque area with a pair of forceps, no difficulty will 
be found in so floating the blastoderm, as to turn it 
upside down, and thus to replace it on the slide with 
the under surface uppermost. 

The points which most deserve attention in this 
view, are : 

1. The heart : its position, its union with the vitelline 
veins, its arterial end. 



442 PRACTICAL DIRECTIONS. [APP. 

2. The fold of the splanchnopleure marking the hind 
limit of the gut ; the vitelline veins running along 
its wings. 

3. The mesoblastic somites on each side of the neural 

canal behind the heart; farther back still, the ver- 
tebral plates not divided into somites. 

F. The examination of the embryo as an opaque 
object. 

This should never be omitted. Many points in 
the transparent embryo only become intelligible after 
the examination of it as an opaque object. 

Having removed the putty ring and coverslip, if 
previously used, allow the blastoderm so far to be- 
come dry, that its edge adheres to the glass slide. 
Care must of course be taken that the embryo itself 
does not become at all dry. Place the glass slide 
with the blastoderm extended flat on it, in a shallow 
vessel containing a solution of picric acid (I. B.). 

If the blastoderm be simply immersed by itself in 
the picric acid solution, the edges of the opaque 
area will curl up and hide much of the embryo. The 
method suggested above prevents these inconveni- 
ences. 

The embryo thus hardened and rendered opaque 
by immersion in the acid (a stay of 2 to 3 hours in 
the solution will be sufficient) may be removed to a 
watch-glass, containing either some of the solution, or 
plain water, and examined with a simple lens, imder 
a strong direct light. The compound microscope will 
be found not nearly so advantageous for this purpose 
as the simple lens. A piece of black paper placed 
under the watch-glass, will throw up the lights and 



APP.] SURFACE VIEW. 443 

shadows of the embryo, with benefit. The watch- 
glass should have a flat bottom; or a shallow flat 
glass cell should be used instead. 

a. Looking at the embryo from above, observe : 

1. The head-fold ; the head distinctly projecting from 
the plane of the blastoderm, and formed chiefly by 
the forebrain and optic vesicles. 

2. The elevation of the medullary canal, and the 
indications of the side walls of the embryo. 

3. The indications of the tail. 

4. The Amnion partly covering the head. Tear it 
open with needles. Observe its two folds. 

b. Having turned the blastoderm upside down, 
observe the following points, looking at the embryo 
from below. 

1. The hinder limit of the splanchnopleure in the 
head-fold, marking the hind limits of the fore- 
gut. The opaque folds now conceal the head almost 
entirely from view. 

2. The commencing tail-fold, and the shallow boat- 
shaped cavity (of the alimentary canal) between it 
and the head-fold. 

The student should not fail to make sketches 
of the embryo, both as a transparent, and as an 
opaque object, seen from below as well as from 
above. These sketches will be of great service to 
him when he comes to study the sections of the 
same embryo. 



444 PRACTICAL DIRECTIONS. [APP. 

G. The following transverse sections will perhaps be 
the most instructive. 

Manipulation as in I. B. 3. 

1. Through the optic vesicles, shewing the optic 
stalks. 

2. Through the hind-brain, shewing the auditory 



"o 
sacs. 



3. Through the middle of the heart, shewing its re- 

lations to the splanchnopleure and alimentary canal. 

4. Through the point of divergence of the splanch- 
nopleure folds, shewing the venous roots of the 
heart. 

5. Through the dorsal region, shewing the medullary 
canal, mesoblastic somites and commencing cleavage 
of the mesoblast. 

6. Through a point where the medullary canal is still 
open, shewing the mode in which its closing takes 
place. 

Longitudinal sections should also be made and 

o 

compared with the transverse sections. 

III. Examination of an Embryo of about 4850 hours. 

A. Opening the egg as in II. A. 

B. Examination of the blastoderm in situ. 
Observe 

1. Thejform of the embryo, which is much more dis- 
tinct than at the earlier stage. 

2. The beating of the heart. 

3. The general features of the circulation. 



APP.J TRANSPARENT EMBRYO. 445 

C. Removal of the Embryo from the yolk, as in 

II. C. 

D. Surface view of the transparent embryo from 

above. 
Notice : 

1. General form of the embryo. 

a. Commencing cranial flexure. 

b. The tail and side folds. 

2. Amnion. Notice the inner and outer (false amnion) 
limbs and remove them with a needle. When the 
amnion has been removed the features of the 
embryo will be much more clearly visible. 

3. The organs of sense. 

a. Eye. Formation of the lens already nearly 
completed. 

b. Auditory involution, now a deep sac with a 
narrow opening to the exterior. 

4. The brain. 

a. The vesicles of the fore-, mid-, and hind-brsiui. 

b. The cerebral vesicle. 

c. The cranial flexure taking place at the mid- 
brain. 

E. Transparent embryo from below. 
Manipulation as in II. E. 

Notice : 

1. The increase of the head-folds of the somatopleure 
and splanchnopleure, especially the latter, and the 
commencement of these folds at the tail. 



446 PRACTICAL DIRECTIONS. [APP. 

2. The now as-shaped heart ; for further particulars 
vide Chap. iv. 

3. The commencing 1st and 2nd visceral clefts arid 
the aortic arches. 

4. The circulation of the yolk sac, vide Fig. 36. Make 
out all the points there shewn and ascertain 
by examination that what have been called the 
veins and arteries in that figure, are truly such. 

F. The embryo as an opaque object, 
Treatment as in. II. F. 

FROM ABOVE : 

Observe the amnion, which is a very conspicuous 
object, and remove it with needles if not done pre- 
viously. The external form of the brain and the 
auditory sac appear very distinctly. 
FROM BELOW : 

Observe the nature of the head- and tail-folds, 
which are much more easily understood from the 
opaque than from the transparent embryos. 

Observe also the alimentary canal, the widely 
open hind end of the fore-gut, and the front end of 
the as yet very short hind-gut. 

G. Sections. 
Manipulation as in I. B. 3. 

The more important sections to be observed, are 
1 . Through optic lobes, shewing : 

a. The formation of the lens. 

b. The involution of the primary optic vesicle. 

c. The constriction, especially from above, of the 
optic stalk. 



APP.] THIRD DAY EMBRYO. 447 

2. Through auditory sac, shewing : 

a. Auditory sac still open. 

b. The thin roof and thick sides of the hind-brain. 

c. Notochord. 

d. Heart. 

e. Closed alimentary canal. 

3. Through dorsal region, shewing the general appear- 
ance of a section of an embryo at this stage, which 
should be compared with a similar section of the 
earlier stage. 

It shews : 

a. The commencement of the side folds; the ali- 
mentary canal still however open below. 

b. The "Wolffian duct lying close under the epiblast 
on the outside of the mesoblastic somites. 

c. The notochord with the aortse on each side. 

IY. Examination of an Embryo at the end of the third 

day. 

A. Opening the egg, as in II. A. 

B. Examination of the blastoderm in situ. 
Observe : 

1. The great increase of the vascular area both in size 
and distinctness. The circulation is now better 
seen in situ than after the blastoderm has been 
removed. 

2. That the embryo now lies completely on its left 
side and that it is only connected with the yolk-sac 
by a somewhat broad stalk. 



448 PRACTICAL DIRECTIONS. [APP. 

C. Removal of the embryo. See II. C. 

It is now unnecessary to remove the whole of the 
blastoderm with the embryo ; indeed it is better to 
cut away the vascular area unless it is wanted for 
examination. 

D. Surface view of the transparent embryo. 

Since the embryo now lies on its side we shall 
not have to speak of the view from above and below. 
The views from the two sides differ chiefly as to the 
appearance of the heart. 

The embryo (freed from the blastoderm and the 
amnion) is to be floated on to a glass slide in the 
usual way. It is necessary to protect it while under 
examination, with a coverslip, which must not be 
allowed to compress it. To avoid this, we have found 
it a good plan to support the coverslip at one end 
only, since by moving it about when thus supported, 
a greater or less amount of pressure can be applied 
at will to the object. 

The details which can at this stage be seen in a 
transparent embryo are very numerous and we re- 
commend the student to try and verify everything 
shewn in Fig.* 37. Amongst the more important and 
obvious points to be noticed are 

1. The increase of the cranial flexure and the body- 
flexure. 

2. The condition of the brain. The mid-brain now 
forms the most anterior point of the head. 

The fore-brain consists of the inconspicuous 
vesicle of the third ventricle and the two large 
cerebral lobes. 



APR] OPAQUE EMBRYO. 449 

The hind-brain consists of a front portion, the 
cerebellum with a thickened roof; and a hinder 
portion, the fourth ventricle with a very thin and 
delicate roof. 

3. Organs of sense. 

The eye especially is now in a very good state 
to observe. The student may refer to Fig. 51, 
and the description there given. 

The ear-vesicle will be seen either just closing 
or completely closed. 

4. In the region of the heart attention must also be 
paid to : 

a. The visceral clefts. 

b. The investing-mass, Le. the growth of mesoblast 
taking place around the end of the notochord. 

c. The condition of the heart. 

5. In the region of the body the chief points to be 
observed are : 

a. The increase in the number of the somites. 

b. The Wolfflan duct, which can be seen as a streak 
along the outer side of the hinder somites. 

c. The attantois, which is now a small vesicle lying 
between the folds of the somatopleure and 
splanchnopleure at the hind end of the body, but 
as yet hardly projects beyond the body cavity. 

E. The embryo as an opaque object. 
Preparation as in II. F. 

The general form of the embryo can be very satis- 
factorily seen when it is hardened and examined as an 
opaque object; but the most important points to be 
F. & B. 29 



450 PRACTICAL DIRECTIONS. [APP. 

made out at this stage in the hardened specimens are 
those connected with the visceral clefts and folds and 
the mouth. 

If the amnion has not been removed it will be 
necessary to pick it completely away with needles. 
Without further preparation a view of the visceral 
folds and clefts may be obtained from the side ; but 
a far more instructive view is that from below, in 
order to gain which the following method may be 
adopted. 

Pour a small quantity of melted black wax (made 
by mixing together lampblack and melted wax) into 
a watch-glass, using just enough to cover the bottom 
of the glass. While still soft make a small depression 
in the wax with the rounded end of a pen-holder or 
handle of a paint-brush and allow the wax to cool. 
In the meantime cut off the head of the hardened 
embryo by a sharp clean transverse incision carried 
just behind the visceral clefts, transfer it to the 
watch-glass and cover it with water or spirit. By a 
little manipulation the head of the embryo may now 
be shifted into the small depression in the wax, 
and thus be made to assume any required position. 
It should then be examined with a simple lens 
under a strong reflected light, and a drawing made 
of it. 

When the head is placed in the proper position, 
the following points may easily be seen. 

1. The opening of the mouth bounded below by the 
first pair of visceral folds, and commencing to be 
enclosed above by the now very small buds which 
are the rudiments of the superior maxillary pro- 
cesses. Compare Fig. 56. 



FOURTH DAY EMBRYO. 451 

2. The second and third visceral arches and clefts. 

3. The nasal pits. 

F. Sections. Manipulation as in I. B. 3. 
The most important sections are : 

1. Through the eyes in the three planes, vide Fig. 50, 
A. B. C. 

2. Through the auditory sac. 

3. Through the dorsal region, shewing the general 
changes which have taken place. 

Amongst these, notice 

a. The changes of the mesoblastic somites: the com- 
mencing formation of the muscle -plates. 

b. The position of the Wolffian duct and the forma- 
tion of the germinal epithelium. 

c. The aortce and the cardinal veins. 

d. The great increase in depth and relative diminu- 
tion in breadth of the section. 

V. Examination of an Embryo of the Fourth Day. 

A. Opening the egg, as in II. A. 

Great care will be required not to injure the 
embryo, which now lies close to the shell-membrane. 

B. Examination in situ. Observe: 

1. The now conspicuous amnion. 

2. The allantois, a small, and as yet hardly vascular 
vesicle, beginning to project from the embryo into 
the space between the true and the false anmion. 

3. The rapidly narrowing somatic stalk. 

292 



452 PRACTICAL DIRECTIONS. [APP. 

C. Removal of the embryo, as in II. C. and IV. C. 

The remarks made in the latter place apply with 
still greater force to an embryo of the fourth and 
succeeding days. 

D. Surface mew of the transparent embryo. For 
manipulation, vide IV. D. 

The points to be observed are : 

1. The formation of the fifth, seventh, and ninth 
cranial nerves. 

To observe these, a small amount of pressure 
is advantageous. 

2. The formation of the fourth visceral cleft, and the 
increase in size of the superior maxillary process. 

3. The formation of the nasal pits and grooves. 

4. The great relative growth of the cerebral lobes and 
the formation of the pineal gland from the roof of 
the vesicle of the third ventricle. 

5. The great increase in the investing mass. 

6. The formation and growth of the muscle-plates, 
which can now be easily seen from the exterior. 

7. The allantois. Make out its position and mode of 
opening into the alimentary canal. 

E. The embryo as an opaque object. Manipulation 
as II. F. For mode of examination vide 
IV. E. 

The view of the mouth from underneath, shewing 
the nasal pit and grooves, the superior and inferior 
maxillary processes and the other visceral folds and 
clefts, is very instructive at this stage. Compare 
Fig. 69. 



APP.] TWENTY HOURS EMBRYO. 453 

F. Sections. Manipulation as in I. B. 3. 
The most important sections are, 

1. Through the eyes. 

2. Transverse section immediately behind the visceral 
arches, shewing the origin of the lungs. 

3. Transverse section just in front of the umbilical 
stalk, shewing the origin of the liver. 

4. Transverse section at about the centre of the 
dorsal region, to shew the general features of the 
fourth day. Compare Fig. 68. 

Amongst the points to be noticed in this section, are 

a. Muscle-plates. 

b. Spinal nerves and ganglia. 

c. Wolffian duct and bodies. 

d. Miiller's duct. 

e. Mesentery. 

f. Commencing changes in the spinal cord. 

5. Section passing through the opening of the allan- 
tois into the alimentary canal. 

For the points to be observed in embryos of 
the fifth and sixth days, the student must consult 
the chapters devoted to those days. 

In the hardened specimens, especial attention 
should be paid to the changes which take place in 
the parts forming the boundaries of the mouth. 

VI. Examination of a Blastoderm of 20 hours. 

A. Opening the egg, as in II. A. 

B. Examination in situ. 

It will not be found possible to make out anything 
very satisfactory from the examination of a blasto- 



454 PRACTICAL DIRECTIONS. [APP. 

derm in situ at this age. The student will however 
not fail to notice the halones, which can be seen 
forming concentric rings round the blastoderm. 

C. Removal of the embryo. 

Two methods of hardening can be adopted at 
this age. One of these involves the removal of the 
blastoderm from the yolk, as in II. C. In the other 
case, the yolk is hardened as a whole. If the latter 
method be employed, the embryo cannot be viewed 
as a transparent object. 

In the cases where the blastoderm is removed 
from the yolk, the manipulation is similar to that 
described under II. C, with the exception of more 
care being required in freeing the blastoderm from 
the vitelline membrane. 

D. Surface view transparent, from above. 
Observe : 

1. The medullary groove between the two medullary 
folds, whose hind ends diverge to enclose between 
them the end of the primitive groove. 

2. The head-fold at the end of the medullary groove. 

3. The one or two pairs of mesoblastic somites flanking 
the medullary groove. 

4. The notochord as an opaque streak along the floor 
of the medullary groove. 

E. Surface view transparent \ from below. 

Same points to be seen as from above, but less 
clearly. 



APP.] TWENTY HOUKS EMBRYO. 455 

F. Embryo as an opaque object. 

As an opaque object, whether the embryo is hard- 
ened in situ or after being removed from the yolk, 
the same points are to be seen as when it is viewed 
as a transparent object, with the exception of the 
notochord and mesoblastic somites (vide D). The 
various grooves and folds are however seen with far 
greater clearness. 

G. Sections. 

Two methods of hardening may be employed ; 
(1) with the embryo in situ, (2) after it has been 
removed. 

To harden the blastoderm in situ the yolk must 
be hardened as a whole. After opening the egg either 
leave the yolk in the egg-shell or pour it out into a 
Berlin capsule ; in any case freeing it as much as 
possible from the white, and taking especial care to 
remove the more adherent layer of white which im- 
mediately surrounds the yolk. 

Place it in picric acid or a weak solution of chromic 
acid (first of '1 p.c. and then of '5 p.c.) with the 
blastoderm uppermost and leave it in that position 
for two or three days. 

Care must be taken that the yolk does not roll 
about ; the blastoderm must not be allowed to alter 
its position : otherwise it may be hard to find it when 
everything has become opaque. If at the end of the 
second day the blastoderm is not sufficiently hard, 
the strength of the solution, if chromic acid be used, 
should be increased and the specimen left in it for 
another day. 

After it has become hardened by the acid, the 
yolk should be washed with water and treated sue- 



456 PRACTICAL DIRECTIONS. [APR 

cessively with weak and strong spirit, vide I. B. 
After it has been in the strong spirit (90 p.c.) for two 
days, the vitelline membrane may be safely peeled off 
and the blastoderm and embryo will be found in 
situ. The portion of the yolk containing them must 
then be sliced off with a sharp razor, and placed in 
absolute alcohol. 

The staining, <fec. may be effected in the ordinary 
way. 

If osmic acid, which we believe will be found 
serviceable for these ear]y stages, is employed, it will 
be necessary to remove the blastoderm from the yolk 
before treating it with the reagent. 

The following transverse sections are the most im- 
portant at this stage : 

1. Through the medullary groove, shewing 

a. The medullary folds with the thickened meso- 
blast. 

b. The notochord under the medullary groove. 

c. The commencing cleavage of the mesoblast. 

2. Through the region where the medullary folds 
diverge, to enclose the end of the primitive groove, 
shewing the greatly increased width of the medul- 
lary groove, but otherwise no real alteration in 
the arrangement of the parts. 

3. Through the front end of the primitive groove 
with the so-called axis cord underneath it, while 
on each side of it are still to be seen the medul- 
lary folds. 

4. Through the primitive groove behind this point, 
shewing the typical characters of the primitive 
groove. 



APP.] UNINCUBATED BLASTODERM. 457 

VII. Examination of an unincubated Blastoderm. 

A. Opening the egg. Vide II. A. 

B. Examination of the blastoderm in situ. 

Observe the central white spot and the peripheral 
more transparent portion of the blastoderm and the 
halones around it. 

C. Removal of the blastoderm. Vide VI. C. 

With the unincubated blastoderm still greater care 
is required in removal than with the 20 hours' blasto- 
derm, and there is no special advantage in doing so 
unless it is intended to harden it with osmic acid. 

D. Surface view transparent from above. 
Observe the absence of the central opacity. 

E. Surface view transparent from underneath. 
Nothing further to be observed than from above. 

F. As an opaque object. 

There is nothing to be learnt from this. 

G. Sections. 
Manipulation as in VI. G. 
The sections shew 

a. The distinct epiblast. 

b. The lower layer cells not as yet differentiated 
into mesoblast and hypoblast. 

c. The thickened edge of the blastoderm. 

d. The segmentation cavity and formative cells. 



458 PRACTICAL DIRECTIONS. [APP. 

VIII. Examination of the process of Segmentation. 

To observe the process of segmentation it will be 
found necessary to kill a number of hens which are 
laying regularly. The best hens lay once every 24 
hours, and by observing the time they usually lay (and 
they generally lay pretty regularly about the same 
time), a fair guess may be made beforehand as to 
the time the egg has been in the oviduct. By this 
means a series of eggs at the various stages of seg- 
mentation may usually be obtained without a great 
unnecessary sacrifice of hens. For making sections, 
the yolk must in all cases be hardened as a whole, 
which may be done as recommended in VI. G. 
Chromic acid is an excellent reagent for this and 
it will be found very easy to make good sections. 

In the sections especial attention should be paid, 

1. To the first appearance of nuclei in the segments, 
and their character. 

2. To the appearance of the horizontal furrows. 

3. As to whether new segments continue to be formed 
outside the limits of the germinal disc, or whether 
the fresh segmentation merely concerns the already 
formed segments. 

4. In the later stages, to the smaller central and 
larger peripheral segments, both containing nuclei. 

For surface views, the germinal disc, either 
fresh or after it has been hardened, can be used. 
In both cases it should be examined by a strong 
reflected light. The chief point to be noticed is 
the more rapid segmentation of the central than of 
the peripheral spheres. 



APP.] STUDY OF BLOOD-VESSELS. 459 

IX. Examination of the later changes of the Embryo. 

For the later stages, and especially for the deve- 
lopment of the skull and the vascular system of the 
body of the chick, it will be found necessary to dissect 
the embryo. This can be done either with the fresh 
embryo or more advantageously with embryos which 
have been preserved in spirit. 

If the embryos are placed while still living into 
spirit a natural injection may be obtained. And such 
an injection is the best for following out the arrange- 
ment of the blood-vessels. 

Sections of course will be available for study, 
especially when combined with dissections. 

X. Study of the development of the Blood-vessels. 

Observations on this subject must be made with 
blastoderms of between 30 40 hours. These are to 
be removed from the egg, in the usual way (vide II. 
A. and C.), spread out over a glass slip and examined 
from below, vide II. E. 

The blastoderm when under examination must be 
protected by a coverslip with the usual precautions 
against pressure and evaporation, and a hot stage 
must also be employed. 

Fresh objects so prepared require to be examined 
with a considerable magnifying power (400 to 800 
diameters). From a series of specimens between 30 
and 40 hours old all the points we have mentioned 
in Chapter iv. p. 92, can without much difficulty be 
observed. 

Especial attention should be paid in the earlier 
specimens to the masses of nuclei enveloped in pro- 
toplasm and connected with each other by proto- 



460 PKACTICAL DIRECTIONS. [APP. 

plasmic processes; and in the later stages to the 
breaking up of these masses into blood corpuscles 
and the conversion of the protoplasmic processes 
into capillaries, with cellular walls. 

Blastoderms treated in the following ways may 
be used to corroborate the observations made on the 
fresh ones. 

With gold chloride. 

Immerse the blastoderm in gold chloride (-5 p.c.) 
for one minute and then wash with distilled water 
and mount in glycerine and examine. 

By this method of preparation, the nuclei and 
protoplasmic processes are rendered more distinct, 
without the whole being rendered too opaque for 
observation. 

The blastoderm after the application of the gold 
chloride should become a pale straw colour; if it 
becomes in the least purple, the reagent has been 
applied for too long a time. 

With potassium bichromate. 

Immerse in a 1 p.c. solution for one day and then 
mount in glycerine. 

With osmic acid. 

Immerse in a *5 p.c. solution for half an hour and 
then in absolute alcohol for a day, and finally mount 
in glycerine. 

PKACTICAL DIRECTIONS FOR OBTAINING AND STUDYING 
MAMMALIAN EMBRYOS. 

XI. Animals and breeding. 

For class work the Rabbit is the most convenient 
animal from which to obtain embryos, it will breed 



APP.] MAMMALIAN SEGMENTING OVA. 461 

freely in the early spring months of the year and will 
give ample opportunity for the student to observe the 
exact time when the female is covered. A number 
of does should be kept together in a large pen, and 
two or three bucks in separate small cages also placed 
within the pen ; at the period of heat, the doe should 
be temporarily placed with the buck and the exact 
time of copulation noted, the age of the embryo 
being calculated from that hour. 

XII. Examination of segmenting ova. 

It will be well to mention here that although 
a doe may have been satisfactorily covered, embryos 
are not always obtained from her. A superficial 
examination of the ovaries will determine whether or 
no fertilized ova are present. If ova have been 
recently dehisced from the ovary, the Graafian follicles 
from which they were discharged will be found to be 
of a distinctly red colour. In case no such ' corpora 
lutea ' as they are called are present further search is 
useless. 

A. To obtain ova from i to 60 hours old. 

Cut open the abdomen from pubis to sternum, 
and from the pubis round the thigh of each side, and 
turn back the flaps of the body wall so formed. 
Remove the viscera and observe below (dorsal) the 
single median vagina, from the anterior end of which 
the uterine horns diverge. 

Observe at the anterior end of each uterine horn 
a small much coiled tube, the oviduct (Fallopian 
tube) near the anterior end of which a little below 
the kidney lies the ovary. Cut out the uterus and 
oviduct together and lay them in a small dissecting 



462 PRACTICAL DIRECTIONS. [APP. 

dish. Carefully stretch out the oviduct by cutting 
the tissue which binds it, and separating it from 
the uterus, taking care to obtain its whole length, 
lay it upon a glass slide. 

With the aid of a lens it is frequently possible to 
distinguish the ovum or ova, through the wall of the 
oviduct. In this case cut a transverse slit into the 
lumen of the duct with a fine pair of scissors a little 
to one side of an ovum ; press with a needle upon 
the oviduct on the other side of the ovum, which will 
glide out through the slit, and can be with ease trans- 
ported upon the point of a small scalpel, or what is 
better spear-headed needle. In case the ovum cannot 
be distinguished in the oviduct by superficial obser- 
vation, the latter must be slit up with a fine pair of 
scissors, when it will easily be seen with the aid of an 
ordinary dissecting lens. 

B. Treatment of the ovum. 

The ovum may be examined fresh in salt solution, 
it is however more instructive when preserved and 
stained in the following manner. 

a. Immerse it in a \ p.c. solution of osmic acid for 
5 or even 10 minutes, transfer it thence to 
the picrocarmine solution described above (I). 
After staining the ovum should then be washed 
in distilled water and placed in a weak solu- 
tion of glycerine in a watch-glass half gly- 
cerine, half water. It should be allowed to 
remain thus under a bell jar for several days 
(7 to 14 or longer) in a warm room until the 
water has evaporated. By this means shrinkage 
and distortion are avoided, the glycerine becoming 



APP.] EXAMINATION OF OVUM. 463 

very gradually more and more dense. It should 
be mounted in glycerine in which 1 p.c. formic 
acid has been mixed to prevent fungoid growths. 
Care must be taken that there is no pressure 
upon the ovum this being insured by the inser- 
tion of a couple of slips of paper one on each side 
of the ovum under the cover glass. 
b. Another method of preservation is used, but 
does not appear to us so successful as the one 
already described. It consists of an immersion 
of the ovum for 5 minutes in i to J p.c. osmic 
acid, subsequent treatment with Mtiller's fluid 
for two or three days, and finally mounting in 
glycerine. 

C. Examination of the ovum. 

The most instructive stages to observe are ova of 

a. 18 hours old, when four segmentation spheres 
will be observed. 

b. 36 hours old when the segmentation is more 
advanced and the spheres numerous. 

The chief points to be noted are : 

1. The number and size of the segmentation spheres; 
in each of which, when treated as described in B. a., 
a large deeply stained nucleus will be visible. The 
spheres themselves are also stained slightly. 

2. The presence of one or two polar bodies on the 
outer side of the segments in ova of not more than 
48 hours old: these also are slightly stained. 

3. The zona radiata immediately surrounding the 
segments, and 

4. The thick albuminous coat, marked with con- 
centric rings. 



464 PRACTICAL DIRECTIONS. [APP. 

D. The fully segmented ovum. 70 hours old. 

The fully segmented ovum is found in the uterus 
at its anterior end close to the place where the 
oviduct opens into the uterus. 

To obtain this stage the uterus must be slit open 
and examined carefully with a dissecting lens : the 
ovum will be seen as a somewhat opaque spot on the 
glistening moist mucous epithelium of the uterus. 

It may be treated in the manner described under 
B. a., but the segments being closely pressed to- 
gether their outlines are not rendered distinct by 
this method. A more advantageous mode of treatment 
is the following : wash the ovum rapidly in distilled 
water, and place it in a 1 p.c. solution of silver 
nitrate for about 3 minutes : then expose it to the 
light in a dish of distilled water until it be tinged 
a brown colour. 

The brown colour is due to the reduction of the 
silver, which takes place chiefly in the cement sub- 
stance between the cells and thus defines very exactly 
their size and shape. The ovum may now be treated 
with glycerine and mounted as described in B. 

The points to be observed are : 

1. The division of the segmentation spheres into the 
layers an outer layer of cubical hyaline cells, and 
an inner of rounded granular cells. 

2. The blastopore of van Beneden. 

3. The presence of a thin layer of mucous outside 
the concentrically ringed albuminous coat of the 
ovum. 



APR] BLASTODERMIC VESICLE. 465 

XIII. Examination of the blastodermic vesicle, 72 90 hours. 

A. To obtain tJie embryo see XII. D. 

B. Prepare the ovum either as in XII. B. or D. 

or in picric acid see I. B. i. 

C. Surface view, or in section see I. B. 3. 
Observe : 

1. The great increase in size of the ovum and the 
reduction in the thickness of the membranes. 

2. The flattened layer of outer cells enclosing a cavity. 

3. The rounded cells of the inner mass attached as a 

lens-shaped mass to one side of the vesicle. 

XIV. Examination of a blastodermic vesicle of 7 days, 
in which the embryonic area and primitive streak are 
present. 

A. To obtain the embryo. 

On opening the body cavity the uterus will be 
found to be uniformly swollen and very vascular. 

Remove the uterus and open it carefully with 
fine scissors along the free, non-mesometric edge, 
taking care to keep the point of the scissors within 
the uterus close against its wall. 

Observe 

1. The oval thin-walled vesicles lying at intervals 
on the walls of the uterus. 

2. The presence of the pyriform embryonic area, at 
the posterior end of which is seen the primitive 
streak. 

F. & B. 30 



466 PRACTICAL DIRECTIONS. [APP. 

3. The commencement of the area vasculosa around 
the hind end of the area. This is seen better 
after treatment with picric acid. 

B. Treatment and Examination of the embryo. 

a. Preserve the vesicle in picric see I. B. 1. 
Stain in haematoxylin, cut out the embryonic 
area, leaving a considerable margin, imbed and 
cut into sections. 

b. In transverse sections observe : 

1. At the anterior end of the area the single row of 
columnar epiblast and the single row of flattened 
hypoblast cells. 

2. Immediately in front of the primitive streak be- 
tween these two layers a few irregularly shaped 
mesoblast cells. 

3. Through the middle of the primitive streak, 

a. Several ] ay ers of rounded mesoblast cells attached 
to, and continuous with, the epiblast in the 
middle line, and stretching out laterally beyond 
the edge of the area. 

b. A single layer of flattened hypoblast. 

4. The epiblast outside the embryonic area in the 
form of flattened cells and, except in the region 
around the primitive streak, overlying a layer of 
flattened hypoblast. 

XV. Examination of an eight days 7 embryo. 

A. To obtain the embryo. 

The uterus will be found here and there to be 
swollen. In these swellings the embryos lie; and 



APP.] EIGHT DAYS' EMBRYO. 467 

owing to the fact that the wall of the embryonic 
vesicle is exceedingly thin, and attached to the ute- 
rine wall, they are very difficult to obtain whole. 

Cut the uterus transversely on each side of the 
swellings and pin the pieces so obtained slightly 
stretched out in small dissecting dishes. Cover the 
tissue with picric acid solution and allow it to remain 
untouched for an hour. Then with two pairs of fine 
pointed forceps carefully tear the uterus longitu- 
dinally, slightly to one side of the median line of the 
free side. This operation will necessarily take some 
time, for but a small portion should be done at once, 
the picric acid being allowed time to penetrate into 
that part of the uterus which has been most recently 
torn open. 

With care, however, the student will be able to 
open completely the swelling and will observe within 
the thin walled vesicle. Great care must also be 
exercised in freeing the vesicle from the uterus. 

This dissection should be performed with the aid 
of a dissecting lens. In case the embryonic vesicle 
is burst it will still be possible to extract the embryonic 
area which lies on the mesometric side of the uterus ; 
the area itself is not attached to the uterine walls. 

B. Examination of surf ace view. 
Observe : 

1. The increased size of the embryonic area. 

2. In the anterior region the medullary folds; di- 
verging behind and enclosing between them, 

3. The primitive streak. 

4. The area opaca now completely surrounding the 
embryo. 

302 



468 PRACTICAL DIRECTIONS. [APP. 

C. Examination of sections. 

Prepare and cut into transverse sections as advised 
in XIY. B. 

Notice 

1. In the sections of the anterior region, 

(L The lateral epiblast composed of several layers 
of columnar cells. 

b. The epiblast in the median line one layer thick 
and in the form of a groove (medullary groove). 

c. The lateral plates of mesoblast. 

d. The flattened lateral hypoblast, and columnar 
hypoblast underlying medullary groove (noto- 
chord). 

2. In sections through the anterior end of the primi- 
tive streak. 

Note the continuation of the epiblast, mesoblast 
and hypoblast in the middle line. 

3. In sections through the posterior end of the area 
the same points to be seen as in XIV. B. b. 3. 

XVI. Examination of an embryo about 8 days 12 hours. 

A. Manipulation as in XV. A. 

B. In surface view observe (cf. Fig. 106) : 

1. Area pellucida surrounding embryo, outside which 
is the well marked area vasculosa. 

2. Widely open neural canal, at anterior end dilated, 
and partially divided into the three primary vesi- 
cles of the brain : note the optic vesicles. At the 
posterior end, the sinus rhomboidalis. 

3. The mesoblastic somites, 4 to 8. 



APP.] FCETAL MEMBRANES. 469 

4. The two lateral tubes of the heart, and the com- 
mencement of the two vitelline veins. 

5. The rudiment of the primitive streak. 

6. The commencing head and tail folds. 

7. The commencing folds of the amnion. 
Compare Fig. 106. 



XVII. Examination of the foetal membranes of an embryo 
of 14 days. 

A. To obtain t/ic embryo, with its membranes. 

Manipulate as in XV. A. only dissect under salt 
solution instead of picric acid. 

B. Observe before removing tJie embryo from the 

uterus ; 

1. The attachment of the vesicle to the mesometric 
side of the uterus over a discoidal area, the 
placental area. 

2. The position and form of the placenta. 

C. Remove the embryo with its membranes intact, 
and observe : 

1. the vascular yolk sac, extending completely round 
the chorion with the exception of a comparatively 
small area where 

2. the allantois is situated. The vascularity of the 
allantois. The foetal villi projecting into the 
maternal placental tissue. 



470 PKACTICAL DIRECTIONS. [APR 

D. Separate the membranes from one another with- 
out tearing them, 

and notice : 

1. The embryo surrounded by the amnion. 

2. The allantois; its position dorsal to the embryo; its 
attachment to the chorion ; its circulation. 

3. The flattened yolk sac, ventral to the embryo ; its 
long stalk; its circulation. 

4. The heart. 

E. The embryo in surface view. 
The points to be observed are 

1. The cranial and body flexure, the spiral curvature 
of the hinder portion of the body. 

2. The vesicles of the brain : cerebral hemispheres, 
fore-brain, mid-brain and hind-brain. 

3. The eye, and the ear. 

4. The heart. 

5. The visceral arches and clefts. 

6. The fore and hind limbs, and the tail. 



APR] NOTES. 471 



NOTE A. 

Since writing the account of section-cutting on p. 434, 
we have obtained more experience as to the practical work- 
ing of Messrs. Caldwell and Threlfall's microtome there 
mentioned. We find that it cuts more accurately and better 
than any other microtome with which we are acquainted, 
and can confidently recommend it to investigators and 
teachers with large classes. In the Cambridge Laboratory, 
it is driven by a small water engine and will cut at a rate 
of 500 a minute, without detriment to the sections. 

NOTE B. 

Mr Threlfall, of Caius College, has recently elaborated 
a method of mounting sections which in our opinion has 
many important advantages over the shellac method. It is 
as follows. Make a solution of pure india-rubber in benzine 
or chloroform. Spread a thin film of this on a clean glass 
slide, and allow it to dry. Arrange the sections on the 
film y melt the paraffin ; allow the slide to cool, then 
immerse the slide for a moment in benzoline (liquid 
paraffin), which dissolves the paraffin, and mount in balsam. 
The chief advantages of this method are that the sections 
do not adhere to the india-rubber until warmed, and they can 
be stained after they are fixed on the slide if necessary. 
For the latter purpose, wash the benzoline away with 
absolute alcohol ; treat with weaker alcohol ; stain ; return 
to absolute ; clear with oil of cloves or kreasote, and mount 
in balsam (vide Zoologischer Anzeiger, 1883). 



INDEX. 



Abdominal wall of chick, 281 

Air-chamber, 3 

Albumen : composition of, 3 ; 
arrangement of, in hen's egg, 
3 ; formation of, in hen, 16 ; 
fate of, in hen's egg, 109; of 
incubated egg, 185 

Alimentary canal of chick, 28 33, 
39; of third day and append- 
ages of, 171 185 ; mammalia, 
417421 

Alisphenoid region of chick, 240, 
246 

Allantoic arteries : of chick, 225, 
293, 298; in mammals, 348, 
410413 

Allantoic veins of chick, 228, 287, 
290; of mammals, 342 

Allantoic stalk, 351 

Allantois : of chick, 28 33, 46 
47, 107, 182 185, 277, 280; 
as a means of respiration, 2 32 ; 
pulsation of, 277; of rabbit, for- 
mation of, 331, 353; of human 
embryo, 336340, 355 358 ; 
of mammalia, structure of, 348; 
of marsupials, 352 : of dog, 358 

Alum carmine, to make and use, 

Amnion : of chick, 28 33, 43 46, 
63, 107, 195; of third day, 113, 
276 280; pulsation of, 277, 
278; false, of chick, 46; of 
rabbit, 330, 353; of human 
embryo, 338 340; of mam- 



malia, 343 ; structure of mam- 
malian, 346; of dog, 358 
Amphioxus, spinal cord of, 254 
Annuli fibrosi of birds, 210 
Anterior commissure of cerebral 

hemisphere, mammalia, 381 
Aorta of chick, 224, 292, 298; 

of mammals, 407 
Aortas of chick of second day, 

89, 103 
Aortic arches of chick, 103, 106, 

167; of fourth day, 225, 291 

298 
Apes' placenta, 355 ; histology of, 

363 ; derivation of, 364 
Aqueductus vestibuli of chick, 

158 

Aqueductus sylvii (see iter.) 
Aqueous humour: of chick, 153 

154; of mammalia, 390 
Arbor vitae of birds, 369 
Area opaca of chick, 7, 49, 195 ; 

mesoblast of, 65 ; hypoblast of, 

65 ; vascular portion of, 74 75, 

no; of third day, 109 
Area pellucida : of chick, 8, 49, 55 ; 

of third day, 1 10 ; of mammals, 

328 
Area vasculosa : of mammalia, 

formation of, 342 ; circulation 

of > 343346 

Arteria centralis retinas of mam- 
malia, 387 390 

Arterial system: of chick, 224 
226, 291 303; mammalia, 407 
409 

Arterial arches, mammalia, 407 



474 



INDEX. 



Articulare of chick, 244 
Attachment of ovum in uterus, 

347 

Auditory capsule of chick, 241 
Auditory pits of chick, 81, 101 
Auricles of chick, 84, 102, 229, 

259, 262 
Auricular : appendages of chick of 

second day, 102 ; septum of 

chick, 257 

Avian characteristics, 275 
Azygos vein, mammalia. 412 



Basi-hyal chick, 245 

Basilar: plate, 235 238; mem- 
brane, mammalia, 397 

Basi-occipital region of chick, 237 

Basi-sphenoid of chick, 240, 246 

Basi-temporal bone, chick, 246 

Beak of chick, 249; formation of, 
282 

Biliary ducts of chick, 180 181 

Birds, oviparous, 308 

Bladder : derivation of, in mam- 
mals, 351 ; mammalian, 417 

Blastoderm of chick, 4 ; struc- 
ture of, in unincubated hen's 
e gg> 7 10 ; area pellucida of, 
8; formative cells of, 23, 24; 
extension of, 26, 27; lateral 
folds of, 37 ; head fold of, 27, 
37; tail told of, 29, 37; vas- 
cular area of, 27 ; hypoblast 
f 51; germinal wall of, 52; 
epiblast, 55 ; of third day, 109, 
no 

Blastoderm of mammal, forma- 
tion of layers of, 314325 ; vas- 
cular area of, 326 ; pellucid 
area of, 328; head and tail 
folds, 329 

Blastodermic vesicle, 314 316, 
319 ; outer layer of, 314; inner 
mass of, 314 ; to examine, 465 

Blastopore of mammalian ovum 
(van Beneden's), 314; of chick 
and mammals, see neurenteric 
canal 



Blood islands of vascular area of 
chick, 91 

Blood corpuscles of chick, for- 
mation of, 92 94 

Blood-vessels : of area opaca of 
chick, formation of, 92 94 ; 
development of, practical di- 
rections for study of, 459, 460 

Body cavity : of chick, 39 ; forma- 
tion of, 40, 41 ; posterior medi- 
astinum of, 267 ; of mammalia, 
406 

Body flexure of chick, 196; on 
third day, 116 

Body flexure : in rabbit, 334 ; in 
dog, 334 ; of human embryo, 
239240 

Borax carmine, to make and use, 
430 

Brain: of chick, 117 123, 281 ; 
of mammalia, 367 387 ; divi- 
sions of, 367 ; hind brain, 367 
370; mid brain, 370, 371; fore 
brain, 371385 ; histogeny of, 

385387 
Branchial clefts and arches (see. 

Visceral) 

Breeding mammals for study, 460 
Bronchi, mammalian, 418 
Bronchial tubes of chick, 177 
Bulbus arteriosus of chick, 84, 225, 

229, 257; septum of, 257, 259, 

260 262 ; of mammalia, 407 



Caecum, mammalia, 419 
Canales Botalli (see Ductus Bo- 

talli) 
Canalis auricularis of chick, 257, 

2 59 

Canalis reuniens, 160; auricularis 
of chick, 169, 229; reuniens of 
ear of mammalia, 393 398 

Cardinal veins : of chick, 1 70 ; 284 
285 ; anterior and posterior 
of mammalia, 409 4 1 3 

Carmine, 431 

Carnivora, placenta of, 358 



INDEX. 



475 



Carotid: common artery of chick, 

295, 298; external and internal 

artery, 292, 295 ; of bird and 

mammal, 408 
Carpus of chick, 234 
Cartilage bones, 242 ; of skull of 

chick, 246 

Cerato-hyals of chick, 245 
Cerebellum: of chick, 122, 203, 

368 370 ; of mammalia, 367 

370; ventricle of, 368; cho- 

roid plexus of, 368; pyramids, 

and olivary bodies of, 368 ; 

arbor vitae, flocculi of, 369 ; 

pons varolii of, 369, 370; velum 

medullas ant. 370 
Cerebral hemispheres : of chick, 

117; of mammalia, 376 385; 

ventricles of, 377; lamina ter- 

minalis, 377; corpus striatum, 

378; commissures of, 381 383; 

septum lucidum, 383 ; fissures 

of, 384385 
Cerebral vesicles of chick, 200 ; 

of second day, 79, 100 
Cerebro-spinal canal in chick, 40 
Cerebrum of mammalia, mono- 

tremata, iusectivora, 384 
Chalazae, 4 

Cheiroptera, placenta of, 353 
Chest wall, of chick, 281 
Chorion : of hen's egg, 47 ; of 

mammal, true and false, 348; 

of rabbit, true and false, 353 ; 

of human ovum, 355358; 

of dog, 358 

Chorion leve, 356 358 
Chorion frondosum, 356 358 
Chorionic villi of mammal, 340 
Choroid coat of eye, of chick, 

141 
Choroid plexuses of mammalia, 

368, 380 
Choroidal fissure of chick, 136 

141, 147 149; of mammalia, 

387 

Chromic acid, 427 428 
Cicatricula, 4 
Ciliary : ganglion of chick, 128 ; 

ridges of chick, 142 ; muscles, 

144 



Circulation : in chick of second 
day, 105; of third day, no 
113; of chick, later stages, 
263 264 

Circulatory system of chick, re- 
sume, 298303 

Clavicle : man, 405 ; of chick, 234 

Clinoid ridge, posterior, chick, 
240 

Clitoris, mammalia, 417 

Cloaca of chick, j 74 ; mammalia, 
418 

Cochlea of chick, 203 

Cochlear canal, mammalia, 390 
398 

Cock, coni-vasculosi, parepidi- 
dymis and vas deferens of, 224 

Columella of chick, 166, 245 

Commissures of spinal cord, 253, 
256 

Coni-vasculosi of cock, 224 

Cornea of chick, 150 153; of 
mammalia, 390 

Cornu ammonis, (see Hippoc. 
major) 

Coracoid of chick, 234 

Coronary vein, mammalia, 409 

413 

Corpora bigemina of chick, 121 

Corpora mammilaria, 378 

Corpora quadrigemina of mam- 
malia, 370; geniculata, 371 

Corpus albicans, 373 

Corpus callosum : mammalia, 381 ; 
rostrum of, 383 ; of marsupials, 
383 ; of monotremes, 383 

Corpus luteum, 311 

Corpus striatum, mammalia, 378 

Corrosive sublimate, how to use, 
426 

Cotyledonary placenta, derivation 
of, 364 

Cotyledons, 359 

Cranial flexure : of chick, 1 16, 196 ; 
of second day, 101 ; of rabbit, 
333; of human embryo, 338 

Cranial nerves : of chick, 123 129, 
203 ; of second day, 101 ; de- 
velopment of, 127 129; of 
mammalia, 400 

Cranium of chick, 235 242 ; 



476 



INDEX. 



cartilaginous, 242 ; cartilage 
bones of, 242 ; membrane bones 
of, 242 

Cranium, mammalia* 401 

Crura cerebri, 371 

Crypts of placenta, 360 363 

Cumulus proligerus, 310 ' 

Cupola, 397, 398 



Decid.ua : of human placenta, 356 ; 

reflexa in human, 356 358 ; 

vera, 356 358; serotina, 356 

358; reflexa in dog, 359^ 
Deciduate placenta, 352 ; histology 

of, 360 

Dentary bones, 246 
Dentine, mammalia, 421 
DESCEMET'S membrane, chick, 151 
Diaphragm, muscles of, 211; 

mammalia, 406 
Diffuse placenta, 359 ; histology 

.of, 360 

Discoidal placenta, 353 
Dog, placenta of, relation with 

placenta of rabbit, 358 
Dorsal aorta of chick, 167 
Ductus arteriosus, man, 408 
Ductus cochlearis of chick, 159 
Ductus Botalli of chick, 287, 289, 

296; of mammalia, 408 
Ductus Cuvieri of chick, 170, 228, 

284 
Ductus venosus of chick, 169, 226 ; 

of mammalia, 413 
Duodenum of chick, 172 174 

E 

Ear: of chick, 156 161 ; of mam- 
malia, 390 397 ; accessory 
structures of, 397 399 

Egg tubes of Pfliiger, 222 

Egg membranes of mammal, 310 

Egg, to open, 437, 438 

Elephas, placenta of, 358 

Embryo of chick : directions for 
examining, 439459 ; of 36 
48 hours, 437 444; of 48 to 
50 hours, 444 447 ; of third 



day, 447 45 1 ; of fourth day r 
451 453; of 20 hours, 453 
456; before incubation, 457; 
segmentation, 458; blood-ves- 
sels of, 459 

Embryo of mammals : directions 
for examination of, 461 470 ; 
of segmenting ova, i 72 hours, 
461 464; of blastodermic vesi- 
cle of, 72 90 hours, 465 ; of 7 
days, 465 ; of 8 days, 466 ; of 
8 days 12 hours, 468 ; of 14 
days, 469 ; of foetal mem- 
branes, 469 

Embryonic area of rabbit, 317; 
composition of, 317 

Embryonic membranes: in mam- 
malia, ideal type, 342 352 ; 
yolk sac of, 345 351 ; amniori 
of, 345 351 ; allantois of, 345 
351; zona radiata of, 345; se- 
rous membrane of, 345 ; cho- 
rion of, 345 ; shedding of, at 
birth, 351; monotremata, 352; 
marsupialia, 352 ; rodentia, 
353, 354 ; insectivora, 353 ; 
cherioptera, 353 ; man and 
apes, 355 358; carnivora, 358 ; 
hyrax, 358; elephas, 358 ; oryc- 
teropus, 358, horse, '359 ; pig, 
359 ; lemurs, 359 

Embryonic sac in chick, 37 38 

Embryonic shield of chick, 49, 
5254 

Enamel, 421 

Endolymph, mammalia, 396 

Epiblast : formation of, in chick, 
25, 26; derivation of, 26; of 
rabbit embryo, 316 ; histological 
differentiation of, in chick, 271; 
epidermis, 271; nervous system, 
271 ; sense organs, 272 ; mouth, 
272 ; anus, 272; pituitary body, 
272; salivary glands, 273; of 
blastoderm from 8th to* 1 2th 
hour, 55 

Epididymis, mammalia, 415 

Epiotic of chick, 246 

Epithelioid lining of heart of 
chick, 88 

Epithelium of throat of chick, 182 



INDEX. 



477 



Epoophoron, of hen, 224 

Ethmoid : region, chick, 240 ; 
lateral, 241 ; bone, chick, 246 

Eustachian tube: of chick, 165; 
of rabbit, 334; of mammalia, 
397418 

Eustachian valve : of heart of 
chick, 263 4 

External auditory meatu* of mam- 
malia, 398 

External carotid artery, chick, 225 

Eye; of chick, 200; development 
of, 132 155 ; of mammalia, 

387390 

Eyelids, of chick, 155; of mam- 
malia, 390 



Face of chick, 246; of human 

embryo, 340 

Facial nerve (see Seventh) 
Falciform ligament, mammalia, 

420 

Fallopian tubes, mammalia, 415 
False amnion of chick, 46 
Falx cerebri mammalia, 377 
Fasciculi teretes, 368 
Feathers, formation of, 282 
Female pronucleus, 17 
Femur, chick, 234 
Fenestra ovalis, of chick, 166, 245 ; 

mammalia, 398 
Fenestra rotunda of chick, 166, 

245 ; mammalia, 398 
Fibula, chick, 234 
Fifth nerve of chick, 126 129, 

203 

Fifth ventricle of man, 383 
First cerebral vesicle of chick, 

second day, 97 
Fissures of spinal cord, 254 
Flocculi of cerebellum of birds, 369 
Foetal appendages : of chick, 276 
280; amnion, 276 278; allan- 
tois, 277; yolk-sac, 277; mem- 
branes of mammal, to examine, 



'oldm ; 



Foldmg-off of embryo chick, 113, 

1 96 
Follicle, ovarian, 12 15 



Foramen ovale : of heart of chick, 
262, 264, 289, 297, 302 

Foramen of MONRO, 372 

Fore brain : of chick, 100 ; of rab- 
bit, 329; of mammalia, 371 
385; optic vesicles of, 387 390; 
thalamencephalon, 371 376 ; 
cerebral hemispheres, 376 
385 ; olfactory lobes, 385 

Foregut of chick, formation of, 
8182 

Formation of the layers in mam- 
mals, 314 325 

Formative cells, 23 24 

Fornix, mammalia, 381 ; pillars 

of, 383 
Fourth ventricle, chick, 122 ; 

mammalia, 368 
Fourth nerve, chick, 128 
Fretum Halleri, chick, 229 
Frontal bones, chick, 246 
Fronto nasal process, chick, 165, 

202, 246 

G 

Gall-bladder of chick, 181 
Gasserian ganglion, chick, 128 
Generativeglands : of chick, 220 

224; of mammalia, 414 415 
Generative organs, external, mam- 
malia, 415 417 
Genital cord, mammalia, 415 ' 
Genital ridge, chick, 220 
Germ cells, primitive, of chick, 

221 

Germinal disc of chick, 12 
Germinal epithelium, 213 
Germinal layers of chick, 26 
Germinal vesicle of chick, 1 2 
Germinal wall, 52 ; structure of, 

65 66; function of, 66 
Glomeruli of kidney of chick, 

214 
Glands, epidermic, of mammalia, 

366 
Glomerulus of Wolffian body of 

chick, 191 
Glossopharyngeal nerve (see Ninth 

nerve) 
Gold chloride, 460 



478 



INDEX. 



Graafian follicle, chick, 222, 310 
Grey matter, of spinal cord of 
chick, 253; of brain of mam- 
malia, 387 

Growth of embryo of chick, 70 
Guinea-pig, structure of blasto- 
derm of, 323; relation of em- 
bryonic layers of, 323; inver- 
sion of the layers in, 341 

H 

Haematoxylin, to make and use, 
429 

Hairs, 365 

Hardening reagents, 425 428; 
picric acid, 425 ; corrosive sub- 
limate, 426 ; osrnic acid ; 427 ; 
chromic acid, 427 ; absolute 
alcohol, 428 ; the necessity of, 
428 

Head of chick, 200 ; of rabbit, 
331 

Headfold of chick, 2729, 33 37; 
16 to 20 hours, 60 ; 20 to 24 
hours, 66 ; of second day, 77 ; 
of mammal, 329 

Heart of chick, 229 230, 256 
264; formation of, 82 89, 102 ; 
beating of, on second day, 89 ; 
of third day, 167; auricles, 
2 p 262 ; ventricles, 260 262 ; 
auricular septum, 257 262; 
ventricular septum, 2 57 ; canalis 
reuniens, 257 259; bulbus ar- 
teriosus, 257 262 ; foramen 
ovale, 262 264 ; Eustachian 
valve, 263 264; circulation in, 
263 264; structure of, 287 
289, 293' 297 ; resume of, 299 

303 

Heart of mammals, 329; struc- 
ture of, 331 ; formation of, 406 ; 
comparison of, with birds, 407 
Hemiazygos vein, mammalia, 412 
Hen: formation of albumen in, 
1 6 ; ovarian follicle of, 12 15 ; 
mesovarium of, 1 1 ; ovary of, 
1 1 ; ovarian ovum of, 1 1 , 15; 
oviduct of, 15; epoophoron, 
paroophoron and oviduct, 224 



Hen's egg, albumen of, 3, 16; 
blastoderm, 7 10, 26, 27 : 
chalazae, 4 ; cicatricula, 4 ; im- 
pregnation of, 17 ; laying of, 
17; polar bodies of, 17; seg- 
mentation of, 1 8 24; vitelline 
membrane of, 4, 13 15 ; yolk 
of, 4 7 ; chorion of, 47 ; shell 
of, i, 16; irregular develop- 
ment of, 48, 49 ; segmentation, 
cavity of, 50 

Hepatic cylinders of chick, 1 79 ; 
circulation of chick, 227 ; veins, 
288290 

Hind brain: of chick, 100 ; of 
rabbit, 329 ; of mammals, and 
birds, 367 370 ; medulla of, 
367 ; cerebellum of, 367 370 

Hippo-campus major, mammalia, 
380 

Hippo-campal fissure of cerebrum 
of mammalia, 385 

Histological differentiation, in 
chick, 269 273 ; of epiblast, 
269, 271; of hypoblast, 269; 
of mesoblast, 269 

Histology of placenta, 359 

Holoblastic segmentation, 307 

Human embryo: villi of, 335; 
early stages of, 335 ; allantois 
of, 336 340; yolk-sac of, 336 
340 ; medullary plate of, 337 ; 
amnion of, 338 340; cranial 
flexure of, 338 340; limbs of, 
339; body flexure of, 339 
340; face of, 340; relation of, 
with other mammals, 341 ; pla- 
centa of, 355 

Human ovum, size of, 307 

Human placenta, histology of, 
363 ; derivation of, 364 

Humerus, chick, 234 

Hyaloid membrane, chick, 144, 
146 

Hyoid arch of chick, 243 245 ; 
of rabbit, 334; of mammalia, 
403404 

Hyoid bone of chick, 245 

Hypoblast of chick : formation of, 
2 5> 5 1 ' 59 > derivation of, 26; 
of area opaca, 65 ; histological 



INDEX. 



479 



differentiation of, 269; of di- 
gestive canal, 272 ; of respira- 
tory ducts, 272 ; of allantois, 
273; notochordal, 273 
Hypoblast of rabbit embryo, 316, 

Hypoblastic mesoblast of chick, 
59 62; of mammal, 321 

Hypogastric veins : chick, 289 ; 
mammalia, 411 413 

Hypohyal, mammalia, 403 

Hypophysis cerebri (see Pituitary 
body) 

Hyrax, placenta of, 358 



Ischium, chick, 234 
Island of Eeil, 385 
Iter a tertio ad quartum ventricu- 
lum, 121, 370 



Jugal bones, chick, 246 
Jugular vein, 284 290 

K 

Kidney: of chick, 218220; tu- 
bules of, 219 ; of mammalia, 
414 



Ileum, chick, 234 
Iliac veins, mammalia, 411 413 
Imbedding, methods of, 432 434 
Impregnation of hen's egg, 17; 

of ovum of mammal, 310 312 
Incubators, makers of, and how 

to manage, 423 
Incus, mammalia, 398, 404 
Inferior cardinal veins, chick, 228 
Infundibulum : chick, 119 121; 

ventricle of, 373 ; tuber cinereum 

of, 373 ; of mammalia, 372 ; of 

birds, 372 
Inner mass of segmented ovum, 

314 ; of blastodermic vesicle, 

3i4 

Innominate artery of chick, 296 

8 

Insectivora, placenta of, 353 
Intercostal veins, mammalia, 

411413 

Interhyal ligament, 403 
Intermediate cell mass of chick, 

95, 189, 190 

Internal carotid artery, chick, 225 
Inter-nasal plate, chick, 240 
Inter-orbital plate of chick, 240 
Intervertebral ligaments, mam- 
malia, 400 
Intervertebral regions, chick, 207, 

209 

Intestine, mammalia, 419 
Inversion of the layers, 341 



Labia majora, mammalia, 416 
Lacrymal bones, chick, 246 ; ducts, 
chick, 155, 156; glands, chick, 
r 55> 156; groove, chick, -248; 
duct, mammalia, 390 
Lagena, chick, 1^9; birds, 397 , 

39? 

Lamina, dorsalis of chick, 29, 62 
Lamina spiralis, mammalia, 397 
Lamina terminalis, mammalia, 

377 . 

Large intestine of chick, 174 
Larynx of chick, 177 
Lateral folds of blastoderm of 

chick, 37 ; of chick of second 

day, 96 

Lateral plates of mesoblast, 68 
Lateral ventricles of chick, 117 : 

of mammalia, 377 ; cornua of, 

T 378 

Laying of eggs, 17 

Lecithin, 6 

Legs of chick, 200 

Lens, chick, formation of, 134, 

149 
Ligamenta suspensoria, of birds, 

2IO 

Ligamentum, pectinaturn, 144 ; 

vesicae medium, 351 
Ligamentum longitudinale an- 

terius and posterius, mammalia, 

402 



480 



INDEX. 



Limbs, of chick, 198 200, 233 ; 
of rabbit, 334 ; of human em- 
bryo, 339 ; mammalia, 406 

Liver of chick, 178 181 ; mam- 
malia, 419 

Lumbar veins, mammalia, 412 

Lungs of chick, 176 178, 267 ; 
mammalia, 418 



M 

Male pronucleus, 17 

Malleus, 398, 404 

Malpighian corpuscles, chick, 182 ; 
bodies of chick, 190 

Mammalia, two periods of develop- 
ment, 308 ; viviparous, 308 

Mammary glands, 366; a source 
of nutriment for the embryo, 
308 

Man (see Human embryo) 

Mandible, chick, 246 

Mandibular arch, chick, 242 
244; maxillary process of, 
chick, 243; rabbit, 334; mam- 
malia, 403404 

Manubrium of malleus, 403 

Marsupialia, foetal membranes of, 

352 

Marsupium, 308 
Maturation of ovum of mammal, 

310 

Maxilla bones, chick, 246 
Maxilla-palatine bones, chick, 246 
Maxillary, processes of mandibu- 

lar arch of chick, 243 
Meatus auditorius externus, of 

chick, 1 66; of mammal, 397 
Meatus venosus, of chick, 169, 

287 
Meckelean cartilage, chick, 244; 

mammalia, 403 
Medulla oblongata, of chick, 122 ; 

of mammalia, 367 
Medullary canal, of chick, 40, 62, 

96 
Medullary folds, of chick, 40, 62, 

66, 77, 97 ; of mammal, 327 



Medullary groove, of chick, 29, 

62 65 ; of rabbit, 320, 32 1 ; 

of man, 338 ; closure of, in 

mammal, 327 331 
Medullary plate, of chick, 62 ; of 

rabbit, 320 ; of man, 338 
Membrana capsulo pupillaris of 

mammalia, 387 389 
Membrana limitans externa, 145; 

granulosa, 310 
Membrana propria of follicles, 

chick, 182 
Membrane : of shell of hen's egg, 

T ; serous, of chick, 32 41 ; 

vitelline of hen's egg, 13 15 
Membrane bones, 242 ; of skull, 

chick, 246 

Membrane of Keissner, mamma- 
lia, 397 

Membrane of Descemet, 389 
Membrane of Corti, and tectoria 

mammalia, 395 
Membranous labyrinth, chick, 

15.8 

Meniscus of birds, 210 
Meroblastic segmentation, 18 
Mesenteric veins of chick, 228, 

288290 

Mesentery, of chick, 173; mam- 
malia, 419 20 

Mesoblast: derivatives of, in chick, 
25 26; of primitive streak of 
chick, 54, 57; derived from 
lower layer cells in chick, 55, 
57, 59 ; of area opaca in chick, 
65 ; splitting of, in chick, 68 ; of 
trunk of embryo chick, 185 
189 ; histological differentiation 
of, in chick, 269; of primitive 
streak of rabbit, 320; of mam- 
mal, double origin of, 321 
323; vertebral zone of, 328; 
lateral zone of, 328 ; somites 
of, 328 

Mesoblastic somites, formation of 
in chick, 70; of chick, 81, 185 
187, 204 208 

Mesocardium of chick, 88 ; forma- 
tion of, 264 

Mesogastrium, chick, 182 
Mesonephros of chick, 212 



INDEX. 



481 



Mesovarium of fowl, 1 1 
Metacarpus, chick, 234 
Metadiscoidal placenta, histology 

of, 362 ; derivation of, 364 
Metamorphosis of arterial arches, 

bird and mammalia, 408 
Metanephos (see Kidney) 
Metanephric blastema, of chick, 

219 
Microtomes, and makers of, 434 

435; 47 1 

Mid brain: of chick, 100, 200; of 
rabbit, 329 ; of mammalia, 370 
371 ; ventricle of, 370; nates 
and testes of, 371; corpora 
geniculata, and crura cerebri of, 

37 1 
Monotremata, foetal membranes 

of, 352 
Mouse, inversion of the layers in, 

34 J 

Mouth, chick, 249, 281 ; of rabbit, 
formation of, 334 

Miillerian duct : chick, 214 2 1 8 ; 
mammalia, 414 415 

Muscle plates of chick, 187 189, 
204 208, 21 1 ; segmentation 
of, 212 

Muscles: hyposkeletal, chick, 211 ; 
episkeletal, chick, 211; cuta- 
neous, chick, 2 1 1 ; extrinsic and 
intrinsic of limb, chick, 212 

Muscular walls of heart of chick, 
88 

N 

Nails, of chick, 283 

Nares : posterior, chick, 251; an- 
terior and posterior, of mam- 
malia, 399 

Nasal capsule, chick, 242 ; car- 
tilages, chick, 246; bones, chick, 
246; groove, chick, 246; pro- 
cesses of chick, inner, 248; 
outer, 248; labyrinth, chick, 
24951 

Nasal organ (see Olfactory organ) 

Nasal pits, of birds, 71 ; chick, 
202 

Nates of mammalia, 371 

F. &B. 



Nerves, of chick of second day, 
101 ; of mammalia, 400 

Nervous system of mammalia, 
367400 

Neural band, chick, 123; crest, 
126 

Neural canal of chick, 31 39, 66; 
second and third day, 122 ; de- 
velopment of, 251 256 

Neurenteric canal, of chick, 71 
74, 175; mammalia, 399; of 
mole, 326, 328 

Ninth nerve, chick, 126 129, 203 

Node of Hensen, 319 

Non-deciduate placenta, 352 

Nose, chick, 249 

Nostrils, chick, 251 

Notochord: of chick, 29, 60 62, 
208 210, 237 238; of second 
day, 101; sheath of chick, 208; 
of mammal, 323, 400; forma- 
tion of, 325 

Nuclei, 1 6 

Nucleolus, 13 

Nucleus, 13 

Nucleus of Pander, 7 

Nucleus pulposus, of birds, 210, 
401 

Nutrition of mammalian embryo : 
308 ; by means of placenta, 350 





Occipital: supra-, basi-, ex-, of 
chick, 246 ; foramen, chick, 237 

(Esophagus of chick, 173 ; mam- 
malia, 418 

Olfactory organ of chick, 161 ; 
nerve of chick, 162 ; grooves, 
chick, 202 ; lobes of mammalia, 

385 

Olivary bodies, 368 
Omentum, mammalia, lesser, 420; 

greater, 420 

Opisthotic of chick, 246 
Optic vesicles : of chick of second 

day, 79, 97 ; chick, 133134 : 

formation of, 141 144 ; of 

rabbit, 329 

31 



482 



INDEX. 



Optic lobes, chick, 121 

Optic nerves, chick, 133, 146 

Optic cup, 134 

Optic chiasma, chick, 147; mam- 
malia, 372 

Optic thalami of mammalia, 373 

Orbitosphenoid, 246 

Orbitosphenoidal region, chick, 
240 

Organ of Corti, mammalia, 395 

Organ of Jacobson, mammalia, 

399 

Orycteropus, placenta of, 358 

Osmic acid, how to use, 427 

Osseous labyrinth, chick, 158 

Otic vesicle, chick, 157 

Outer layer, of blastodermic vesi- 
cle, 314 

Ova, primordial, of chick, 221 

Ovarian follicle : of hen, 12 15 ; 
mammal, 309 

Ovarian ovum: of hen, n 15; 
of mammals, 309 

Ovary: of adult hen, n ; of 
chick, 222; of mammals, 
309 ; follicles of, 309 ; corpus 
luteum of, 311. 

Oviduct of adult hen, 15 ; of 
chick, 224 

Oviparous animals, 308 

Ovum : of birds and mammals 
compared, 307 ; of mammal 
in follicle, 309 ; membranes of, 
310; maturation and impreg- 
nation of, 310 312; polar 
bodies of, 311 ; segmentation 
of, 312 314; blastopore of 
(Beneden), 314 



Palate, mammalia, 420, 421 
Palatine bones, chick, 246 
Pancreas: of chick, 181 ; mam- 
malia, 419 

Pander, nucleus of, 7. 
Parachordals, chick, 235 238 
Paraffin, 432434 
Parepididymis of cock, 224 
Parietal bones of chick, 246 



Parieto-occipital fissure of cere- 
brum of man and apes, 385 

PARKER on the fowl's skull, 245 

Paroophoron of hen, 224 

Pecten, chick, 147 

Pectoral girdle, chick, 234; mam- 
malia, 405 

Pelvic girdle, chick, 234 ; mam- 
malia, 405 

Penis, mammalia, 417 

Pericardial cavity, chick, develop- 
ment of, 264 269 ; of rabbit, 
331; mammalia, 406 

Perilymph, mammalia, 396 

Periotic capsules, chick, 237 

Peritoneal covering of heart of 
chick, 88 ; cavity, mammalia, 
406 

Peritoneum, mammalia, 4 19 420 

PFLUGER, egg tubes, 222 

Phalanges, chick, 234 

Pharynx, mammalia, 418 

Picric acid, how to use, 425 

Picro-carmine, to make and use, 

.43 1 

Pig, placenta, histology of, 360 

Pineal glands, chick, 117 119; 
of mammalia and birds, 373 
376 

Pituitary body : chick, 119 1 2 1 ; 
rabbit, 334; of birds, 372; 
mammalia, 372, 420 

Pituitary space, chick, 240 

Placenta : 342 ; discoidal, deci- 
duate, type of, 353, 354; meta- 
discoidal, type" of, 354358 ; 
decidua of, 356; chorion laeve 
of, 356 358; chorion frondo- 
sum of, 356 358 ; comparison 
of, 358; zonary type of, 358; 
diffuse form, 359 ; polycotyle- 
donary form, 359 ; histology of, 
3593^3; evolution of, 364; 
of sloth, 360. 

Pleural cavity, chick, development 
of, 264 269 ; mammalia, 406 

Pleuroperitoneal space of chick, 
28 33, 84; formation of, 40, 
41, 68 

Pneumogastric nerve (see Tenth 
nerve) 



INDEX. 



483 



Polar bodies, 1750! ova of mam- 
mals, 3 r i 
Polycotyledonary placenta, 359 ; 

histology of, 360 
Pons Varolii of birds, 369 ; of 

mammals, 370 
Position of embryo chick of third 

and fourth days, 113 116 
Postanal gut, of chick, 175; of 
rabbit, relation of, to primitive 
streak, 329 

Posterior nares, chick, 202 
Potassium bichromate, 460 
Premaxilla bones, chick, 246 
Prenasal bones of chick, 246 
Presphenoid region, chick, 240 

T, ? 46 

Primitive groove of chick, 56 ; of 

rabbit, 320 
Primitive streak of chick, 52 62 ; 

of chick from 20 to 24 hours, 

70; of rabbit, 319 
Processus infundibuli, chick, 121 
Proctodasum of chick, 175; of 

mammal, 422 
Pronephros, 218 

Pronucleus, female, 17; male, 17 
Prootic, chick, 246 
Protovertebrae (see Mesoblastic 

somites) 

Pterygo-palatine bar, chick, 243 
Pterygoid bones, chick, 246 
Pubis, chick, 234 
Pulmonary veins of chick, 228, 

289 290 
Pulmonary arteries of chick, 294 

298; mammalia, 407 
Pupil, chick, 142 
Pyramids of cerebellum, 368 

Q 

Quadrato-jugal bones, 246 
Quadrate, chick, 243 

E 

Kabbit embryo, growth of, 327 

334; placenta of, 353 
Badius, chick, 234 



Eat, inversion of the layers in, 

34i 
Eecessus labyrinthi, mammalia, 

39 39 8 
Eecessus vestibuli (see Aqueductus 

vestibuli) chick, 203 
Eespiration of chick, 303 ; of third 

day, no 

Eete vasculosum, mammalia, 4 14 
Eetina, chick, 142, 144146 
Eibs, chick, 234; mammalia, 405 
Eodentia, placenta of, 353 
Eods and cones of retina, chick, 

146 

Eostrum, chick, 246 
Euminants' placenta, histology of, 



Sacculus hemisphericus, mam- 
malia, 390 398 
Salivary glands, mammalia, 420 
Scala media (see Cochlear canal) 
Scala tympani, mammalia, 395 

397 

Scala vestibuli, mammalia, 395 

397 
Scapula of chick, 234 

Sclerotic coat of eye of chick, 141 

Sclerotic capsules, mammalia, 405 

Scrotum, mammalia, 416 

Sebaceous glands, 366 

Secondary optic vesicle (see Optic 
cup) 

Sections, method of cutting, 434 
436 ; mounting of, 436 

Segmentation: of hen's egg, 18 
24; meroblastic, 18; of mam- 
malian ovum, 312 314; of 
hen's egg to observe, 458; of 
mammalian ovum to observe, 
461 

Semicircular canal : of chick, 
158 ; mammalia, 390 398 

Semi-lunar valves, chick, 258 

Sense capsules of chick, 211 212 

Septum lucidum, mammalia, 383 

Septum-nasi, chick, 246 

Serous membrane of chick, 32 



484 



INDEX. 



Serous envelope of chick, 107 ; 

of mammals, 346 
Seventh nerve of chick, 127 129, 

203 

Shell-membrane of chick, i 
Shell of hen's egg, i ; formation 

of, 16 

Shield, embryonic, of chick, 49 
Sinus rhomboidalis : of embryo 

chick, 71, 81 ; of rabbit, 329 
Sinus terminalis, of chick of 

second day, 91, 104; in rabbit, 

343 

Sinus venosus of chick, 169, 226, 

285 290 

Skeleton of limb, chick, 234 
Skull of chick, 235 251 ; cartilage 
and membrane bones of, 246 ; 
of mammalia, 401 405 
Sloth, placenta, histology of, 360 
Somatic stalk of chick, 29 42 ; 

of mammals, 351 
Somatopleure of chick, 29 33; 

formation of, 4041, 68 
Spermatozoa of chick, 223 
Spinal nerves : of chick, 123; de- 
velopment of, 129132 ; of 
mammalia, 400 

Spinal cord of chick: develop- 
ment of, 251 256 ; white mat- 
ter of, 252; grey matter of, 
253; canal of, 252 256; epi- 
thelium of, 251, 252; anterior 
grey commissure of, 256 ; an- 
terior fissure of, 254 256; 
dorsal fissure of, 255 256; 
posterior grey commissure of, 
256; sinus rhomboidalis of, 
256; anterior columns of, 256; 
posterior columns of, 256 ; 
lateral columns of, 256; an- 
terior white commissure of, 
256; posterior white commis- 
sure of, 256 
Splanchnic stalk of chick, 29 

42, 232 
Splanchnopleure of chick, 29 

33 ; formation of, 40 42, 68 
Spleen of chick, 182 
Splint bones of chick, 246 
Squamosal bones of chick, 246 



Staining reagents, 428 432; has- 
matoxylin, 429 ; borax carmine, 
430; carmine, 431; picro-car- 
mine, 431 ; alum carmine, 431 

Stapes, of chick, 245 ; mammalia, 
398, 404 

Sternum of chick, 235 ; of mam- 
malia, 405 

Stomach of chick, r 73 ; mam- 
malia, 418 

Stomodaeum, of chick, 119, 203; 
mammalia, 420 

Stria vascularis, mammalia, 397 

Subclavian arteries of chick, 296 
298. of mammalia, 409 

Subclavian veins, mammalia, 409 

413 

Sulcus of Monro, 373 

Superior maxilla of chick, 165 ; 
maxillary processes of chick, 
202; of rabbit, 334 

Superior cardinal veins of chick, 
228 

Supra-renal bodies, mammalia, 
structure of, 413; relation of, 
with sympathetic nervous sys- 
tem, 414 

Subzonal membrane of mammal, 

34 6 

Sylvian fissure, mammalia, 384, 

385 
Sympathetic nervous system of 

mammalia, 400 
Sweat-glands, 366 



T. 

Tail-fold of chick, 29 37, 196; 
of second day, 96 ; of mammal, 

3 2 9 

Tail-swelling of chick, 74 

Tarsus of chick, 234 

Teeth, mammalia, 421 

Tela choroidea, 375 

Tenth nerve of chick, 125, 127 

129, 203 ^ 

Testis of chick, 222, 371 
Thalamencephalon : of chick, 

117; of mammalia, 371 376; 

ventricle of, 372; floor of, 372, 



INDEX. 



485 



373; sides of, 373; roof of, 374 

376 

Third nerve of chick, 129 
Third ventricle of mammalia, 372 
Throat of rabbit, formation of, 

Thyroid body, of chick, 181 ; 

mammalia, 418 
Tibia of chick, 234 
Tongue of chick, 282 
Trabeculae of chick, 236, 239 241 
Trachea of chick, 176, 177 ; mam- 
malia, 418 
Tuber cinereurn, 373 
Turbinal bones of chick, 246 
Tympanic cavity of chick, 166 ; 
membrane of chick, 166 ; cavity 
of mammalia, 397, 418; mem- 
brane of mammalia, 397 

U. 

Ulna, of chick, 234 

Umbilical, arteries (nee Allantoic); 
veins (see Allantoic veins); vesi- 
cle of mammals (see Yolk-sac) ; 
stalk of chick of third day, 113; 
cord, 351 

Urachus, 351 

Ureter of chick, 219; mammalia, 

4i7 

Urethra, mammalia, 417 

Urinogenital organs of mam- 
malia, 414 417; sinus of mam- 
malia, 415417 

Uterine crypts, 350 

Uterus, mammalia, 415 

Utriculus of mammalia, 393 398 

Uvea of iris, chick, 144 



V. 

Valve of Vieussens, of birds, 369 ; 

of mammals, 370 
Vagina mammalia, 415 
Vagus nerve (see Tenth nerve) 
Vasa efferentia and recta mam- 
malia, 414 

Vascular system of chick, 224 
230; of second day, 89 94, 102 



106 ; of third day, 167 170 ; 
mammalia, 406 413 

Vascular area: of blastoderm of 
chick, 27; of third day, no 
113; of rabbit's ovum, forma- 
tion of, 326 

Vas deferens : of cock, 224 ; mam- 
malia, 415 

Velum medullas anterius (see 
Valve of Vieussens) ; posterius, 

37 
Vermiform appendix, mammalia, 

419 
Vena cava, inferior, of chick, 228, 

285 290 ; mammalia, 409 

413 

Venae cavae, superior, of chick, 
286 290 ; of mammalia, 409 

4i3 

Venae advehentes of chick, 227, 
287 289 ; revehentes of chick, 
227, 287 289 

Vena terminalis (see Sinus termi- 
nalis) 

Venous system: of chick, 226 
229, 283 290,301 303; mam- 
malia, 409 413 

Ventricles of brain of chick of 
second day, 102; of mammals, 
117, 121 122; of chick, 229 

Ventricular septum, chick, 230, 

257 

Vertebrae of chick, primary, 205 
208 ; permanent, 205 208 ; 
bodies of, 207 209 

Vertebral arches, osseous, of 
chick, 207, 210; mammalia, 
409 

Vertebral artery of chick, 295 
298 

Vertebral column, of chick, 205 
208 ; membranous, 205 208 ; 
secondary segmentation of, 205 
208 ; explanation of do., 205 
206 ; of mammalia, early de- 
velopment, ossification of, 400, 
401 

Vertebrate animal, general struc- 
ture of, 39 

Vesicle of third ventricle (see 
Thalamencephalon) 



486 



INDEX. 



Vessels of placenta, 360 363 

Vestibule, chick, 158 

Villi : of human ovum, 335 ; of 
zona in dog, 347; of subzonal 
membrane of rabbit, 347 ; of 
chorion of mammal, 349; of 
placenta, 360363 

Visceral arches, 245 ; of rabbit, 

334 

Visceral arches of chick, 162 167; 

of rabbit, 334; of mammalia, 

402 
Visceral clefts: of chick, 162 

167, 281; closure of do., 164; 

of rabbit, 334; of mammalia, 

402, 418 

Visceral folds of chick, 163 
Visceral skeleton of chick, 242 

246 
Visceral vein of chick, 284 290 ; 

of mammalia, 409 413 
Vitellin, 5 
.Vitelline arteries: of chick, 167, 

293 298, 225; of second day, 

89, 103 
Vitelline duct of chick, 196, 232 ; 

of mammals, 350 
Vitelline membrane, 4; of hen's 

egg, 1 3 1 5 ; of mammal, 310 
Vitelline veins of chick, 84, 226, 

288 290 ; of second day, 92, 

104; in rabbit, 343; of mam- 
malia, 410 413 
Vitreous humour of chick, 140, 150 



Viviparous animals, 308 
Vomer of chick, 246 

W 

White matter : of spinal cord of 
chick, 252; of brain of mam- 
malia, 386 387 

Wings of chick, 200 

Wolffian body: of chick, 190 
193; of mammalia, 4/4; of 
chick of second day, 106 

Wolffian duct of chick, 190, 213 ; 
of second day, 94 95, 106; of 
mammalia, 414 

Wolffian ridge of chick, 198 

Wolffian tubules of chicK, 106, 
191193, 213 



Yolk of hen's egg, 4 7 ; arrange- 
ment of, 6; structure of, 5 

Yolk-sac: of chick, 28 37, 277 
280; of mammals, 327; of 
marsupials, 352; of rabbit, 353; 
of human ovum, 355 358; of 
dog, 358 

Z 

Zona radiata, 310; of chick, 15 
Zonary placenta: histology of, 
360 ; derivation of, 364 



CAMBRIDGE : PRINTED BY C. J. CLAY, M.A. & SON. AT THE UNIVERSITY PRESS. 




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